More information needed to find out why your bookmarks disappeared. You did not ask about recovering your lost bookmarks. Have you already recovered them? Do you need help recovering them?

Regarding how they disappeared... do you use FireFox sync for bookmarks?

Regarding the page that was running slow in FireFox, do you have any idea what was running slow? For instance, I get messages like that when a page with a lot of remote videos have to load through a javascript, especially if the internet connection is slow.

Do you have an "Old Firefox Data" folder on the desktop ?

• https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

when you use the refresh feature in Firefox then a new profile is created and some personal data (bookmarks and history, Form Data, tabs) is automatically imported. The current Firefox profile folder will be moved to an "Old Firefox Data" folder on the desktop. Installed extensions and other customizations (toolbars, prefs) that you have made are lost and need to be redone.

It is possible to recover data from the "Old Firefox Data" folder on the desktop, but be cautious not to copy corrupted files to avoid carrying over problems.

You can copy specific files with Firefox closed to the current profile folder to recover personal data.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data