This question has been answered, apparently satisfactorily, for presumably non-Firefox browsers (see Hot keys to switch between tabs?).
The accepted solution there is ctrl+shift+pg up/down. On Firefox, this merely rearranges browser tabs. Would be grateful for an alternative solution (or way to turn off Firefox).
In terms of hopefully disabling the Firefox shortcut that's theoretically overriding the one given in the accepted answer, I can find neither keyconfig nor 'better touch' in the Firefox add-ons; nor can I find anything built into Firefox relating to specifying keyboard shortcuts.
I am running FF Quantum 57.0.4.
Best Answer
The linked question now has an answer that works in Firefox (confirmed in 70 on Windows). Copyediting: