Hi, it depends what was removed, but for bookmarks only, you can try this: Open bookmarks > Show All Bookmarks > Import and Backup > Restore, and see if you have a backup there.

If not, and for other data, please see -

• Profiles - Where Firefox stores your bookmarks, passwords and other user data.

• Recover important data from an old profile.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

when I open my profile and click on boomarks they are in a JSON file which I don't know how to open.

You can't. In most cases, only the program that creates the data file can open the file.

The best thing you can do is copy all files to an external hard drive. This includes your user profiles.

Plugins would be restored when you re-install its program.