@Charlie74 I've had two close friendships which ended and then recommenced. One I didn't know why the friendship ended - we had a massive argument over nothing and didn't talk for a few years, but later I found out I'd just borne the brunt of other complexities in her life. We rebuilt slowly, worked near each other and would catch up for lunch. At some point, after a few months we had a big heart to heart about what had happened and now several years later we are still the best of friends, closer than ever. The other was my BF from high school, she got married young and became such a biaitch that I exited her life - it was basically her, or all my other friends. Later she got in touch when her marriage failed and I felt for her and we started hanging out and resumed the friendship. A few years on and things went a bit haywire for me with severe glandular fever and I couldn't do what I'd previously done, she was very unsupportive, tried to break up my marriage (she wanted my DH), and I saw that the behaviors I'd attributed to her ex-DH (treating everybody like rubbish), was actually her as well. I ended the friendship and it was very hard as she'd been important yo me, but she wasn't a nice person. I suspect she has a personality disorder and she destroys others.
If if I were you I'd approach with caution. I guess you'd need to access why she has contacted you now and who is she now? What does she want? Has she burnt all her other friends? Putting aside your history, would you choose her as a friend now? I think @tuxcat makes a very valuable point, you are vulnerable right now. I find it tough enough keeping up with existing friends and you don't need extra drama in your life.
You could accept her Facebook request and just quietly see what happens? Something must have triggered her reaching out and I'd want to know what that is before I got too far in.