If you have selected 'always allow' when choosing your devices in Firefox, you will not be asked which device to use in future. To undo this:
1. Go into your Firefox Options
2. Click Privacy
3. Clear Recent History
4. Click 'Clear all current history'
5. In the list of check boxes, ensure 'site preferences' is selected, but no others.
6. Select 'everything' as the time range to clear.
7. Click 'Clear Now'.
From this point, when you join a Firefox meeting, it will ask you what device to use.