I set browser.link.open newwindow = 1 so that all links open in the same tab. I like the consistency of left-click is always same tab and middle-click always new tab as opposed to getting whatever convention the site author decided upon.

However this frequently causes a number of issues:

• Some sites don’t use real <a> tag links so middle-click doesn’t work. Now I can only open in the same tab. I consider these sites broken but it is still annoying as often these sites are also very slow to load so it can be annoying to lose the current page (for example if I want to quickly open a lot of search results in new tabs).
• Some sites try to open a pop-up for login or other authorization but don’t explicitly set a request for a new window so it overwrites the current tab where JS is expected to receive the authorization result. Therefore you can’t do this authorization. This one I can’t really blame the site as much because while they weren’t explicit every other browser would behave as expected here.