In Firefox, on mostly Linux distributions it seems, pressing CTRL+Q (the blockquote-insert shortcut) causes Firefox to quit, and doesn't insert quotes into the question body. You can configure Firefox so that it shows a popup warning you that you are about to close X number of tabs, but it still won't insert the blockquote.
I have tested this in Firefox 81.0 and 80.04, both 64-bit, on Debian Linux Buster, Ubuntu 20.04, and Kali Linux. While this shortcut works for most browsers and OS's, it doesn't for Linux distros.
Feature-request: Change the shortcut so that it doesn't conflict with a shortcut of Firefox's. Many of us use Firefox, and a fair number Linux, myself with a dual-boot setup, so this is kind of annoying.