Firefox

At least on my devices, there's two ways of doing it, either changing the font size.

Or on lock developer settings by clicking on build number in the about phone than Software Information. Than go back into the main menu for setting and at the bottom you shoul see developer settings. Once you enter developer settings scroll to the bottom and you should set find a option called minimum width. decreasing the number makes things more compact while increasing it does the opposite. I would recommend going by small increments of 10 to 50 since going to drastic will make it very difficult to change the setting back.