First of all: people, read the bloody info!
Although you think you get it, although you think you can handle it...
Gorhil is pretty clear (and correct) about it:
*** uMatrix IS NOT FOR EVERYBODY...
RTFM!

As for the coding/ addon: been using it for a while now, and it is perfect! Yes, it breaks stuff sometimes. But then again, that's why you installed it for in the 1st place, right?
Confused? Don't get that last statement?
OK, if you can't figure that out, return to ***
...
Don't say they didn't warn you. ☺

BTW, if you DO know what this is all about, uMatrix works brilliantly with Gorhills (now new) uBO-Scope, because it shows you what those cryptic URLs actually do. So you can block them, if you want. You know, break stuff... Works for me!

Gorhill: I know you don't take feature requests, but concerning uMatrix... If you come over the code (you know, because you're there any way) which defines the name of the backup (settings/ rules) file... Can you please make it add the current date and time, just like with the backup files for uBO? Would be immensely helpful. Enables to keep a small backlog of previous configurations without renaming the files. I know, nothing xdotool on my box can't automate, but still... Cheers...

Brilliant that this is now photon-compatible, thanks.

However, it seems to have lost my previous uMatrix profiles, so several sites refused to render or to work properly (including addons.moz comments!) until I updated them. Ouch.

That's a bit careless, with no notification. But I sort of understand, and it's a little unfair to criticise after all the work Raymond has done. I blame moz more for the dog's breakfast approach.

Very puzzling until I worked it out. And I had been complaining about Nightly developers, too, releasing a version that didn't render properly, when it was the unannounced uMatrix update that did it.