You can check this support article.

• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

You can check for problems with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use the "Places Database" -> "Verify Integrity" button in "Help -> More Troubleshooting Information" (about:support)

If errors are reported with "Verify Integrity", close and restart Firefox or reboot the computer and retry.

Make sure you install Firefox properly and that you do not launch Firefox from the DMG (Disk Image) file.

Open the Firefox Disk Image file and drag the Firefox application to the Applications folder on your hard drive. You shouldn't double-click the Firefox application to run it from the Disk Image, but instead drag it out of the DMG folder.

• https://support.mozilla.org/en-US/kb/install-firefox-mac