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

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP- in front of the numbers/letters, click it to submit them.

The crash report is several pages of data. We need the report numbers to see the whole report.

Using your mouse, mark the most recent 7 - 10 Submitted crash reports, and copy them. Now go to the reply box and paste them in.

For more help on crash reports, see; https://support.mozilla.org/en-US/kb/firefox-crashes-asking-support

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Thank you for the answers, my problem still persists. Looks like firefox is not crashing (but simply closing), since my latest unreported crash is from 2018

Is there a way I can monitor the file that has the settings with the search engines or simply lock the file that has it?

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

I assume you are removing "Browser Default" from the Search panel of the Preferences page -- One-click Search Engines box -- and it is getting reinstalled at the next startup?

This article describes that box in a bit more detail: Change your default search settings in Firefox ("Remove or add search engines" section toward the end).

Assuming that is the situation, the most likely source is external malware. You can use programs listed in the support article FredMcD linked earlier: Troubleshoot Firefox issues caused by malware. Malwarebytes has a free version for Mac.