You should already be able to do this, just select two dictionaries at the same time, as described here: https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker#w_switch-dictionaries If you don't have to needed dictionaries, you can a...
The only issue I have are the Privacy Protection and DoH sections. Having a single "choice" and an advanced settings button makes no sense to me. Those should be similar to the ones at Tracking protection and HTTPS-Only Mode.
Personally I think a white-label e-mail solution would make sense, both for individuals and teams. For example mailbox.org's model is pretty good I think (not an ad, I am a paying customer).
I have tried this in 99.0 and 100.0b3. It works for me in both versions. Are you selecting the tabs by Ctrl-clicking (or Shift-clicking) and dragging? Are the tabs highlighted?
