Firefox has a peculiar way of handling tabs overflow that occurs when users open too many tabs for the tab bar to display properly. By default, Firefox pushes the extra tabs off the screen as new ones get spawned; the tab bar acting as a conveyor belt of sort. Arrows appear on both edges of the tab bar that allows the user to horizontally scroll the tab bar and access the out-of-screen tabs. Personally, I find this behavior disorienting.
It seems the accepted rule for handling tabs overflow, as followed by Opera, Chrome, Internet Explorer and the majority of browsers, is to resize them to accommodate new tabs. So I use the Noverflow add-on (now called Prevent Tab Overflow) to get this behavior in Firefox. This might not be the best way to handle tabs overflow but definitely better than the way Firefox does.
Another way to tackle tabs overflow is to use stacks, the concept introduced by Opera in version 11, where you manually drag and drop tabs over each other to create stacks. Chrome followed suit and added an experimental automatic tab stacking feature. Now tab stacking is available in Firefox through the Tab Stacking add-on.
Tab Stacking automatically stacks tabs on top of each other when the tab bar in Firefox becomes crowded. The add-on does this dynamically by stacking tabs on either far side of the active tab. The tabs goes in and out of the stack as the user changes focus from one tab to the next. There is no permanent stack of tabs. The image below will give you an idea of how this tab stacking works.
The active tab and the three tabs around it are in an expanded state. The tabs immediately next to them are collapsed in a stack. As you can see, there are two stacks on both end of the tab bar. When you change focus to a new tab, the stack changes.
The user can control the minimum width, which is the width of the active tabs and those immediately around it, and the width of the stacked tabs. There is another option to set the maximum width of the stacked tabs.
Tab Stacking is a proof-of-concept add-on in alpha version and may contain several bugs.