11 years after the bug was filed, Mozilla has finally started taking action on https://bugzilla.mozilla.org/show_bug.cgi?id=380637, which directly addresses this issue.
In current versions of Firefox (I'm using v.59) there is a new per-site preference, but unfortunately it's still problematic (as discussed in recent comments on the bug ticket) in v.64: It prevents the Backspace and Del keys from working in the browser itself (e.g. to edit entered text), which is likely to be a deal-breaker for most people.
Update: This is bug https://bugzilla.mozilla.org/show_bug.cgi?id=1445942 which was allegedly fixed last week. Hopefully the fix will show up in FF 65.
But you can experiment with the current implementation (and hopefully, access a fixed version in Firefox 65) by clicking the
(i) icon in the Location Bar (or by pressing Ctrl-I, presuming you're not on a site like Stack Overflow that's hijacked that shortcut), then clicking the arrow of the seemingly unrelated
Connection > menu item.
(Thanks to @PerJohansson for pointing out that they've made this setting much more difficult to find since v.59.)
Now you should see "More information" at the bottom. Click that, and you'll finally get to the
Page Info dialog. Now go to the
Permissions tab, and adjust the Override Keyboard Shortcuts` setting:
But with irresponsible applications (I'm looking at you, Atlassian) taking over your keyboard without asking, I still find this useful. I'd rather be able to use e.g. my PageUp/PageDown and cursor keys to navigate JIRA just as if it were a well-behaved, user-friendly application. It's a bit maddening but not too hard to briefly unblock shortcuts for the site when I need to write/edit a comment.