I've written a web app that uses ctrl+alt+ shortcuts because they don't interfere with the many expected ctrl- shortcuts. Librewolf blocks these shortcuts because of RFP, apparently. As a result I have to disable RFP for my site (or just be annoyed all the time), and I guess I'll have to advise my users to do the same.

I haven't found any doc that explicitly discusses the justification for disabling alt shortcuts. There is a bug about how ctrl- was broken for a while. This page doesn't mention alt either, that I can find.

I can guess that it probably allows identifying some keyboards somehow. Just would like to read up on it so I can at least understand what the issues are with the alt key.