What you can do is to go to the Mozilla Add-ons Web Page and see if these work on your system;

FEBE

This will save your add-ons in the event of trouble,

and help you easily put them back.

OPIE

This will save add-on and user settings and can restore them.

Firefox data is stored in a hidden folder:

~/Library/Application Support/Firefox/Profiles/<profile folder>

One way to open that folder is from the Troubleshooting Information page. Either:

• Help > Troubleshooting Information
• type or paste about:support in the address bar and press Return

Look for the "Show in Finder" button in the first table.

This article describes the files most important to rescue, but I agree with FredMcD that a comprehensive backup is best in case you forget something or the article has not been updated to cover everything. Good luck!

Restore bookmarks, passwords and data from an old Firefox profile

P.S. When copying and when transplanting these files, it's best to exit Firefox to release file locks and make sure that pending changes are fully written out.