Mac OS uses the command and shift keys together for some things. For example:
Along with the "]" or "[" keys it moves from one tab to the next.
Along with the "w" key it closes all tabs.
I keep having a problem with these things in Firefox. I practically never want to close all tabs. However, I move between tabs a lot.
I often have a long list of tabs which I've arranged in a queued sequence and deal with them progressively. Sometimes it takes me a long time to prepare such a sequence. I move to a tab by holding the command and shift keys with one hand and pressing "]" or "[" with the other. I do this a lot while examining and comparing tabs in a sequence.
When I've finished with one tab I use command-"W" to close it. Every so often I lose concentration and don't release the command and shift keys, which I've been holding down with the other hand, and the result is, all tabs are closed.
I know, this is macOS behaviour, and Firefox needs to follow those conventions, even when they're problematical. However, it would help me if I got a dialogue box asking me to confirm when I'm going to close more than one tab. I get that when quitting Firefox. It would be a great help to have it too when I'm about to close multiple tabs.
With best wishes and thanks to your team for making this helpful app available to us mac users.