When browsing with a large number of open pages (especially if you are a heavy tab user), closing tabs accidentally is common. This happens very often because Firefox has close buttons on each tab, and when a large number of tabs are present, the proximity between those red close buttons get very small.
To prevent accidental closing or modification of tabs (for example, when clicking the option “Close other tabs”), Permatabs Firefox extension comes to your rescue. This nifty extension does the equivalent of stickies on forums – it turns tabs of your choice (selectively) to permanent tabs, which cannot be closed or modified. These tabs also stay around between sessions, which means that you do not lose them when you close Firefox and restart it again.
Permatabs extension for Firefox
The color of permanent tabs can be tweaked (Tools> Addons> Permatabs> Options), and you can also force links inside permanent tabs to open in new tabs (so that you do not lose the tab when clicking on links).
Also see Vertigo extension, which enables you to stack up Firefox tabs vertically.