Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

Starting DE doesn't crash. So I guess there is an issue with my addons & 124.0.1. Is there an easier way to start my existing firefox profile and on/off extensions until finding the problem ? I'm rather unwilling to refresh wholesale, having a decent amount of extensions & their data

As a note, I just downgraded (looked for 124.0 & installed it over my current firefox install folder) and everything is working perfectly. I'm half tempted to stay like this for a short while and see if either the problematic addon or firefox receives an update to fix this issue.

124.0.1 fixed security vulnerabilities marked as "Critical" which is the highest severity level.

If it still happen in Troubleshoot Mode then it is not caused by an extension.

When Firefox is not running, backup your profile folder somewhere safe and try renaming the prefs.js file to see if an about:config setting is responsible.

Closing firefox, going into the profile folder ; renaming prefs.js and prefs-1.js to prefs-3.js into their respective OLDprefs*.js ; then upgrading to 124.0.1 (through the mozilla FTP installer) now allows me to start firefox without it instantly crashing. About Firefox confirms me i'm on 124.0.1. However something is deeply wrong: no websites can load. They just attempt to load indefinitely. Can't right click, opening network/console/debugger is possible through shortcuts but doesn't display anything. Half my addons icons occupies space but have no icon on the menu bar, and the browser is overall quite broken. I can manage extensions, turned most of them off, restarting the browser for no results.

And now for what is hopefully that last magic trick, I merely closed firefox (124.0.1) ; went back to restore my OLDprefs*.js into prefs*.js ; and started firefox again. It's working. No issue I can notice, I had some extensions turned off before closing that I reactivated now with no issue, About Firefox does confirm i'm on 124.0.1, and everything does seem in order. I guess doing those manipulations somehow fixed the issue, I will be restarting the computer and check again.

So, yeah, seems working on my end. I guess running firefox with fresh prefs.js before putting mine back it somehow solved it. Thanks for the help