You mean UI issues with Firefox, compared to Safari? Gosh, where to start?
- Standard keyboard shortcuts for text editing still don't work. That alone is a dealbreaker for me.
- The toolbar(s) don't look or work like standard toolbars. The standard toolbar editor is missing, too.
- Tabs don't look or act right, either. They're actually styled like Windows XP tabs.
- The scroll bars look good, but scrolling doesn't work the same as any other app. It hits a solid wall at either end.
- The preferences dialog is not a dialog, but a web browser tab (!), and its controls all look strange. They're not like standard OS controls or standard web controls. They're mostly just words floating on empty space, and some of them happen to be clickable. (It doesn't even have the advantage of being a webpage: the back button is enabled, but don't take me back to the last thing I was looking at here.) Some settings open a (custom, of course) dialog-in-a-tab, which is resizable, but if you resize it, the main scrollbar disappears, so you can't see the bottom of the dialog with the Cancel/OK buttons.
- It doesn't use the standard localization setting. It has its own, but half of Firefox doesn't even seem to use that, because even though my System Preferences is English, and my Firefox language preference is English, and even my Firefox content preference is English, it's showing half the UI in Japanese still. The context menu is half English and half Japanese!
- It's got a whole second menu (!) in the toolbar, with no way to hide it (see above: non-standard toolbars). The font, text alignment, highlight color, icons, and animation are all non-standard. Of course, it doesn't obey the system accessibility settings, either.
- Tree controls (e.g., in the History window) are non-standard, and standard keyboard shortcuts don't work here, either.
- It uses a (custom, of course) pop-over as an upgrade notification, which can't be dismissed by clicking outside them. Or dismissed permanently, like normal notifications.
The whole reason I use a Mac is for consistency. That's been its primary benefit since 1984. Instead of learning 10 completely different different programs, I just have to learn the common parts once. (Remember those keyboard overlays for the IBM PC we used so we didn't have to memorize which F-key meant "Save" in each different program?) In Firefox, everything is custom, and works differently than other apps. The keyboard, mouse, display, menus, windows, and controls are all weird. If I were OK with that, I'd switch back to Linux.