Firefox's new tab behavior is a bit inconsistent. Right-click on a link and choose Open Link in New Tab and the new tab opens directly to the right of your current tab. In contrast, if you open a new tab, it opens to the right of all of your open tabs.
Firefox add-on Tab Deque changes this behavior. After installing the add-on and restarting Firefox, new tabs will open next to your current tab. This change alone will keep me using Tab Deque, but it boasts another useful feature that makes it easy to toggle between two tabs. If you click on your current tab, Firefox will take you to the last tab you viewed. And, of course, if you click on a tab that you aren't currently viewing, you'll open that tab. Taken together, these two behaviors let you jump back and forth between two tabs without needing to move your cursor.
Tab Deque also supports keyboard shortcuts. Using Shift and Page-up or -down (that's Shift and the Function key and the up or down arrow on a Mac) lets you jump forward and back through your tabs.
Tab Deque supports other features, but their descriptions on the add-on's page are a bit opaque. Personally, I like Tab Deque solely for the ability to open a new tab right next to the one I'm currently viewing. If you find other uses for Tab Deque, please share in the comments below. And if you have a favorite tab manager for Firefox or Chrome or any browser for that matter, I'd appreciate your commentary as well.