Can't close tabs or open new tabs
I've seen multiple people posting this problem back more than two years, but every question describing the problem is locked, so I can't comment on them. Instead I'm posting my own question...and answer.
I had the same problem - seemingly at random, Firefox will not allow tabs to be closed. Clicking the "X" on the tab is ignored, clicking the "+" to open a new tab is also ignored.
I found the problem is not Firefox at all, but Flash. Frequently when I visit a site that includes Flash (i.e. Youtube or any video site), this problem will occur. If you open your Task Manager, you'll see a "FlashPlayerPlugin" exe running (usually two instances). Terminate that process, and the problem in Firefox goes away.
So the problem is not just with Firefox, it's with the Flash plugin, AND in the way that Firefox manages its plugins.
Opaite Mbohovái (2)
First, try to reset Firefox. Refresh Firefox - reset add-ons and settings. This simple step will eliminate many Firefox problems with only one click.
A few other basic trouble shooting things you can try is this: Run all Windows Updates, install all needed service packs, etc.
Update your graphics driver (Firefox uses your graphics card for some rendering, and an out of date graphics card driver can cause problems. Upgrade your graphics drivers to use hardware acceleration and WebGL
Update all your plugins (Flash, Java, etc.): http://www.mozilla.org/plugincheck/.
Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections.
Thanks for your useless boilerplate answer attempting get yet another "solution" on your tally, but if you had actually READ my question in the first place instead of just copying and pasting your standard answer template, you would have realized that I actually already PROVIDED the solution, and was simply posting it for the information of other users.