Thank you for taking the time to reply to me.
First of all, I just wanna say that you're right!
It is a very recent thing, I tested different versions of FF, and v55 has the same issue, but Firefox 54 works like a charm!
What happens exactly in FF 56 (and 55 apparently, as I just found out), is that even though this add-on changes the favicon successfully, and it shows on screen that the change takes effect immediately, upon page load of that specific bookmark, the favicon reverts to default. FF probably "reads" the favicon from the page, and updates the bookmark, and thus overriding what this add-on does.
BUT, upon browser restart, the icons change back to whatever you assigned them from this add-on.
Sometimes they will change back even if I dont restart the browser (weird, cannot reproduce this), and I dont know why.
I asked for help in Mozilla support, here:
And 2 Top Contributors already confirmed that this is an issue with Firefox and there's no easy way to solve this yet.
You can check the thread if you like.
In any case, its pretty sad that I have to choose between performance (FF 56 is significantly faster) and functionality.
I guess a compromise has to be done somewhere, and I understand that.
The thing is, that this wasnt the only issue I had with FF 56, but I was able to fix the other issues, except this one though.
Anyway, thanks again, if you have any ideas let me know, if not thats fine.
I will be going back to FF 54 for now, but I'm not sure if that will work properly. Guess I'll find out.
EDIT: I've been using FF 54.0.1 all day with no issues until now. I think that this version is truly the last best FF version that supports old XUL addons.
And yes I agree, its not worth the effort.
Even though benchmarks indicate that FF 56 is A LOT faster than FF 54, in practice, the difference is pretty small.
Your add-on works great btw, and I got my favicons back, exactly as I want them. My userscripts work fine, and now I have the added bonus ability of customizing which ever bookmark icon I want!! (thx to this add-on)