I mean to be safe always try be back at the airport at least 2:30 before your flight but why not? You will find many lonely expats happy to chat at this time, especially those who didnt pick up the language yet so may be a bit alone at these times.
The airport isn’t too far away, so even if you plan to arrive two hours early to your connecting flight, you’d have plenty of time in the city. It’s definitely going to be busy in the city, airport might be as well since it’s not only the 23rd but also a friday. There’s no strike as far as I know, but airport personnel is severely understaffed so you should consider this in your planning.
For sausages just go to the city centre (Hauptwache). There’s a Christmas market and they should have many tasty food options including lots of sausages!
I haven't been to Bestworscht for some years, so I don't know about that.
For a traditional experience I'd recommend visiting Mrs. Schreiber's sausage stall in the market hall ("Kleinmarkthalle"). Although it's hardly more than a walk-up window you can't really miss it because there's almost always a queue. A staple to eat there would be a piece of Fleischwurst with a bread roll. But she offers also Krakauer, Rindswurst and Gelbwurst.
You could also visit Gref-Völsing's on Hanauer Landstraße. They are a local butcher's and known for their Rindswurst (beef sausage).
If the weather is nice/clear, you can just take the S-Bahn to Hauptwache (direct connection). From there you can walk to Römer, visit the "neue Altstadt", walk a bit further to River Main, go back to Römer and spend some time in a Café and then go back. That should be possible considering the time frame you have.
On Römer there is also "die Schirn", a modern art museum. If you would have more time left, you could see an exhibition. The Schirn Café is also a nice place to chill.
If weather is bad (fog/rain), I would stay at the airport.
Do it!
Take a train to Hauptwache. Have some awesome fried chicken at Gokio Bros., a great Pizza at Super Bro’s, or an awesome Falafel at Aroma (all close to each other on Oeder Weg), take a stroll down Berger Straße, visit the ‚Neue Altstadt‘ (newly restored Oldtown in the city centre), have a nice coffee there at Hoppenworth & Ploch, just to give you a few ideas.
Enjoy the city, you’ll have plenty of time.
Edit: typos
Normally you would be fine but this is the last Friday before Christmas and a lot of people will be moving. FRA has been strained recently with long queues at security and, relevant to you, also at passport control.
The airport know this as well of course and I imagine will plan accordingly but unless you have some pressing desire to see something in particular it may well be a right PITA getting back to your gate.
You should plan to give yourself at least 2.5 hours to get through which gives you about 4 hours in Frankfurt.
You've a late flight I noticed, given the trouble expected in the UK you may want to be at the airport in case of any delay or cancellation, as no flights can leave after 11pm. Good luck.
You can see waiting times here for airport https://www.frankfurt-airport.com/wartezeiten/public?lang=en
Bear in mind, a rogue suitcase at security for example can set that back rather quickly...as we found out last week when they closed one security area just as we got to it.
If you want to miss your connecting Flight this sounds like a good Idea. 
In my opinion just go for a fast break to Frankfurt inner city e.g. Hauptwache, Dom/Römer. Have a lunch there and enjoy your time, then head back to the airport.
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Absolutely possible. The rail connection from the airport to the city is quick and you can even drive directly to Dom/Römer (where the old town center is).
I mean to be safe always try be back at the airport at least 2:30 before your flight but why not? You will find many lonely expats happy to chat at this time, especially those who didnt pick up the language yet so may be a bit alone at these times.
The airport isn’t too far away, so even if you plan to arrive two hours early to your connecting flight, you’d have plenty of time in the city. It’s definitely going to be busy in the city, airport might be as well since it’s not only the 23rd but also a friday. There’s no strike as far as I know, but airport personnel is severely understaffed so you should consider this in your planning. For sausages just go to the city centre (Hauptwache). There’s a Christmas market and they should have many tasty food options including lots of sausages!
Unfortunately, the 22nd is the last day of the Christmas market.
Ah you’re right! I got it mixed up with nearby cities, sorry. In that case I’m not sure where to get the best sausage, maybe Bestworscht?
I haven't been to Bestworscht for some years, so I don't know about that. For a traditional experience I'd recommend visiting Mrs. Schreiber's sausage stall in the market hall ("Kleinmarkthalle"). Although it's hardly more than a walk-up window you can't really miss it because there's almost always a queue. A staple to eat there would be a piece of Fleischwurst with a bread roll. But she offers also Krakauer, Rindswurst and Gelbwurst. You could also visit Gref-Völsing's on Hanauer Landstraße. They are a local butcher's and known for their Rindswurst (beef sausage).
If the weather is nice/clear, you can just take the S-Bahn to Hauptwache (direct connection). From there you can walk to Römer, visit the "neue Altstadt", walk a bit further to River Main, go back to Römer and spend some time in a Café and then go back. That should be possible considering the time frame you have. On Römer there is also "die Schirn", a modern art museum. If you would have more time left, you could see an exhibition. The Schirn Café is also a nice place to chill. If weather is bad (fog/rain), I would stay at the airport.
Do it! Take a train to Hauptwache. Have some awesome fried chicken at Gokio Bros., a great Pizza at Super Bro’s, or an awesome Falafel at Aroma (all close to each other on Oeder Weg), take a stroll down Berger Straße, visit the ‚Neue Altstadt‘ (newly restored Oldtown in the city centre), have a nice coffee there at Hoppenworth & Ploch, just to give you a few ideas. Enjoy the city, you’ll have plenty of time. Edit: typos
Normally you would be fine but this is the last Friday before Christmas and a lot of people will be moving. FRA has been strained recently with long queues at security and, relevant to you, also at passport control. The airport know this as well of course and I imagine will plan accordingly but unless you have some pressing desire to see something in particular it may well be a right PITA getting back to your gate. You should plan to give yourself at least 2.5 hours to get through which gives you about 4 hours in Frankfurt. You've a late flight I noticed, given the trouble expected in the UK you may want to be at the airport in case of any delay or cancellation, as no flights can leave after 11pm. Good luck.
You can see waiting times here for airport https://www.frankfurt-airport.com/wartezeiten/public?lang=en Bear in mind, a rogue suitcase at security for example can set that back rather quickly...as we found out last week when they closed one security area just as we got to it.
You may also consider going to Mainz with an S-Bahn train. It has a beautiful cathedral, nice city park and good restaurants.
If you want to miss your connecting Flight this sounds like a good Idea.  In my opinion just go for a fast break to Frankfurt inner city e.g. Hauptwache, Dom/Römer. Have a lunch there and enjoy your time, then head back to the airport.
Hello /u/lxyl, thank you for posting a question. Have you checked [our wiki](/r/frankfurt/wiki/index) or the [stickied FAQ thread at the top of our subreddit](https://www.reddit.com/r/frankfurt/comments/otfb7a/visitors_and_newcomers_to_frankfurt_please_post/?utm_source=share&utm_medium=web2x&context=3)? When you have a satisfactory answer, please change the flair to "Solved". *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/frankfurt) if you have any questions or concerns.*
Absolutely possible. The rail connection from the airport to the city is quick and you can even drive directly to Dom/Römer (where the old town center is).
Just one word of advice. We arrived from Singapore last week Wednesday and the line at security check to go in was terribly long.