Firefox may not be shutting down correctly, and for that reason doing a crash recovery at startup. I'm not sure what you changed in about:config, but please try this:

You can switch Firefox from automatically restoring tabs after a crash to instead show you a list of the windows and tabs in the session so you can select only the ones you want back. (Or none of them.) Here's how:

(i) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(ii) In the search box above the list, type or paste sess and pause while the list is filtered

(iii) Double-click the browser.sessionstore.max_resumed_crashes preference and change the value from 1 to 0 (that's a zero) and click OK

Please do not change any other sessionstore preferences without researching or asking about them first.

Hope this helps.

The user.js setting you mentioned also could get embedded in prefs.js, but it should be cleared after being executed. Some users have problems updating prefs.js or it gets rolled back by utility programs with browser settings protection features (e.g., Advanced SystemCare). More info on investigating these problems in this article: How to fix preferences that won't save.

Would you disable the add-on 'Tab Auto Reload' and see if that will make a difference  ?