Under Firefox's Settings is "Zoom" which changes the size of web-content (which you can also change simply by pressing Ctrl-+ or Ctrl-–) but has no effect on the Firefox user interface itself. Firefox has far more settings than appear on the Settings menus though. These are accessed by typing <code>about:config</code> into the search/URL bar. The first thing you'll see is a warning which you have to acknowledge before you get to the actual settings. At the top of the next screen is a search box into which you can type the setting you are wish to change, or just part of it. That will then show up below, or several that match the part of it you entered if you didn't enter the full thing. If you want the tabs, icons &c to be bigger the setting is in <code>about:config</code> and is called <code>layout.css.devPixelsPerPx</code>. The default value is -1.0 so try changing it to 1.2, without the '-.' If that isn't big enough try 1.25 or 1.3 but DO NOT enter a big value — 2 is huge. If you enter something like 20 it will become so big that you won't be able to see it to change it again. This will also increase the size of web-content so if that's now too big you can reduce it with the Zoom setting already alluded to.