You should always save important files before something like that.

Bookmark Backups

Type about:support in the address bar and press enter.

Under the page logo on the left side you will see Application Basics. Under this find Profile Folder. To it’s right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Enter the bookmarkbackups folder. This is where Firefox makes backups of your bookmarks. Look for the jsonlz4 file you want to restore (check the time / date / size). Right click on that file and select Copy. Now go back to the profile folder where you started. Rename the file places.sqlite by adding .old to it. Now right click in the folder and select Paste. The file you just pasted here, rename it to places.sqlite. After you are done, restart Firefox.

Edited to correct file name error.

I you have used Firefox Sync, you just have to login again in your new firefox install [[How to Set Up your Firefox Sync|https://support.mozilla.org/en-US/kb/how-do-i-set-up-firefox-sync]] and you will find all your bookmarks, passwords and other things again. If you haven't use Firefox Sync in your last Firefox session, i'am sorry for about that.

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

FEBE (Firefox Environment Backup Extension) {web link} FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backup of files that you choose.

OPIE {web link} Import/Export extension preferences