What is wrong with computer (see below)?

My computer, or more specifically my internet browser I guess, has been acting strange.

When I right click to open a link in a new tab, sometimes two tabs of the page open instead of one, and sometimes the page I was already on changes to the page I right-clicked on as well.

Drop-down menus are having problems too; I click on them and they flash up for a fraction of a second and go away, and don't stay up unless I click on them three or four times. Not always though, only sometimes.

Highlighting is not working well either. It is hard to explain, but when I hold down the mouse to hightlight some text, either I have to click several times for it to work, or highlights completey different text from what I wanted to, or I can't get it to hightlight the whole line.

Sometimes I will click on thing and it will activate something else - like I click the "x" to close one browser window, and the window behind it closes as well, but only if the two "x's" are aligned.

I have the latest version of firefox; I have updated all my plugins; I have reset firefox; I have run virus scans, so if it is a virus it's something my antivirus isn't detecting. I have googled around a bit and looked a firefox support, but it is a little hard to research a problem with symptoms as varied and subtle as these. What should I do?