[jaharios]

If you can't do that maybe you shouldn't install custom extensions. (Joking, but maybe mozilla thinks that way?)

Anyway if you want to be able to run normal Firefox version (no beta,developer or esr) and want to install custom extensions there is an easier way but not well know.

Firefox Unbranded

https://wiki.mozilla.org/Add-ons/Extension_Signing#Unbranded...

It works fine expect for the "ugly" icon, but it is possible to modify it.

[Wowfunhappy]

The biggest issue with the unbranded builds is that there are no automatic updates.

Mind, it would probably be easier to write your own autoupdater than to runtime patch the signature check in official Firefox...

[jaharios]

Actually I had set a policy to block automatic updates. Sometimes they break things, and having a profile backup before updating to roll back in case of breakage is a must. So it is a win win for me.