Since I upgraded to Firefox 4.0 from 3.6.** my middle mouse button behaves strange. When I press the middle mousebutton to scroll in a website and it happens that I release the mousebutton with the mouse over a link, the link gets opened in a new tab. The same happens with the links in my bookmarks toolbar. I am aware of the functionality of opening links in new tabs with the middle mousebutton, but in pre 4.0 versions of firefox if you wanted to open a link in a new tab, you had to hover the mouse over a link directly and specifically click it with the middle mousbutton. With 4.0 you can press the middle mousebutton anywhere on the website (even if there is no link) and then accidently release it over a link to have firefox open it in a new tab. This is very annoying i.e. if you want to scroll through a gallery with many pictures (that have links to bigger versions of themselves) and accidently releasing the middle mousebutton on a gallery image (with link) instead of some empty space. This way many new tabs that are not wanted get opened in the background. Additionally the functionality to just click the middle mousebutton to scroll and then click again to end scrolling is missing. You now have to keep the middle mousebutton pressed to scroll, which is a little annoying too if you want scroll through large websites/documents and have to keep the middle mousebutton pressed all the time.