Is this with all websites/pages, some, a few?

What were you doing at the time? surfing, watching a clip, reading something . . . .

Does it look like at about the same time? More than once per day?

https://support.mozilla.org/en-US/kb/update-firefox-latest-version?cache=no Did you update Firefox to the latest version?

Version 85.0.2, first offered to Release channel users on February 9, 2021

Note that the OP is on Mac OS X 10.11 and can't update beyond Firefox 78 ESR.

• https://support.mozilla.org/en-US/kb/macos-users-esr

Does Firefox crash and offers to send a crash report or does it merely close ?

Are there possibly memory issues that cause the browser to close ?

• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources