If you're looking for perhaps the most transformative workflow addon for a web browser, Tree Style Tab is undeniably one of the best.
Tree Style Tab is an intuitive way to group your tabs into a visual route of your browsing. If you instead create new tabs each time you jump sites, you'll create a detailed 'tree' of how you found resources and where those resources took you.
The interface is fairly simple, but requires you to adjust where you look for your tabs. Due to Firefox's own self-imposed limitations, Tree Style Tabs can't hide the default tab bar and exists as a sidebar. You can still use the native tab bar, but you can create hassles for yourself by reordering tabs using the native tab bar rather than Tree Style Tab.
That said, Tree Style Tab can really change how you browse the web and task manage on the web. Also, you should probably pair it with an addon like Tab Unloader so you can unload tabs you're not using otherwise hundreds of tabs can bog down Firefox.