Mozilla Firefox (download for Windows, Mac, Linux, Android, or the Portable App) didn't invent tabbed browsing, but they sure as heck perfected it, or at least the extended network of extensions developers did. If Firefox doesn't include a tabbed-browsing feature you want, there's a good chance you can find one among its numerous add-ons.
Personally, I'm a tab glutton. I might be using a Windows XP box from 2004, but that doesn't stop me from running four Firefox windows with 10-15 tabs open in each. Each window usually relates to a specific task, project, or email request, and each tab within each window includes various sorts of specific content, documentation, or Web tools. When researching, I'll generally open every link in a new tab, to scan and close or save as I work through. So tab management is more than a luxury for me, it's a necessity for my job.
From that perspective, while there are thousands of tab-related Firefox extensions, there are only four tab add-ons that I find critical for working in Mozilla Firefox, and to be honest, only one that is absolutely essential.
The once and future king of tabbed-browsing add-ons for Mozilla Firefox, Tab Mix Plus is the biggest reason that I use Firefox as my default browser (Chrome's incompetence in managing Adobe Shockwave crashes is another one). It allows Firefox users to customize the tabbed-browsing experience from soup to nuts--add a progress bar (of any color) to show the loading status of any tab; specify maximum and minimum tab width or the number of tab rows; lock, protect, rename, merge, or duplicate tabs, or send them to new windows, all from completely customizable right-click tab and tab-bar menus.
For me, one of the most valuable features of Tab Mix Plus is the ability to tweak Ctrl-Tab behavior. With Tab Mix Plus, you can set Crtl-Tab to navigate tabs in the most recently used order (like Windows' Alt-Tab) and to add an Alt-Tab preview menu (again, like the Windows OS) that displays screenshots of all your open tabs.
Tab display allows all sorts of customization, letting users differentiate between visited and non-visited tabs with colors, italics, bold, or other display options, a useful mechanism for sorting or organizing tabs. Tab Mix Plus also offers four new option buttons for your browser toolbar: Closed Tabs, which displays a list of your 10 most recently closed tabs; Closed Windows, which does the same thing for your recently closed windogs; Open Tabs, which presents a handy drop-down menu of all open tabs in any window; and the Session Manager, which would be a fantastic add-on all by itself.
Tab Mix Plus's Session Manager lets you default to Firefox's, but when enabled, it can save multiple sessions, automatically save on exit, automatically load on startup, never ever save a session ever, or prompt you as often as you like. Once enabled, the Session Manger becomes a drop-down selection from the Firefox Tools menu, or as mentioned, can be added as a button to the Firefox toolbar. Ctrl-1 saves all the tabs in your current window; Ctrl-9 saves all the tabs in all the current open windows as unique sessions. Much of this functionality can be duplicated with plain old Firefox bookmark groups, but TMP's Session Manager is far more elegant and efficient.
tl;dr: Tab Mix Plus is the bee's knees. I can't imagine using Firefox without it.
This functionality comes default in Google Chrome, and I can't fathom why it's not a default tab-management option by default in Firefox. Here's the use case: you have a Web page or Web tool from which you need to access multiple pages; you want to open them all in new tabs; then read or take action on those tabs; then close the tabs and return to your original site or tool.
If that original site or tool is your only other tab, you're fine. Just right click that tab and select "Close Other Tabs." But what if you have two other tabs to the left that you want to keep open? With Tab Mix Plus, you could "protect" those first two tabs, right-click the third and select "Close Other Tabs," but a more elegant solution is the appropriately named add-on Close Tabs to the Right, which does exactly what it says on the tin--it adds a "Close Tabs to the Right" function to every Firefox tab.
It may seem trivial, but I need it. You need it. Get it.
I'm reaching back here to an add-on that is no longer supported (and hasn't been in a few years now), but there's no other extension that does what NumExt does--which is to add a simple-text numeral to the left of each tab, labeling all of your open tabs 1 to X from left to right in your Firefox tab bar. By default, Firefox allows users to access specific tabs using Ctrl-1 through Ctrl-9; NumExt expands on that functionality to enable Alt-1 through Alt-9 for accessing tab numbers 9-17 (NB: In Firefox, Ctrl-9 always navigates to the last tab to the right, regardless of how many tabs are open).
NumExt still works with the latest stable Firefox release (currently 18.0.1), but it definitely shows the kludginess of an add-on that hasn't been maintained for years--there is a command-line system for managing tabs with key commands (e.g. "close 4" to close tab 4, regardless of which tab you are focused on), but I wasn't able to get it to work in my latest install of NumExt. Also, if you use Tab Mix Plus' "Rename Tab" functionality, you'll lose your NumExt number, at least until a refresh. And if you drag tabs around to put the in different order, NumExt will take some time to get the numbers all sorted out.
At the very least, however, NumExt will show you how many tabs you have open. Tab hoarders who keep 4-5 windows open with countless tabs in each might take the appearance of numerals 20 and above as indication that browser and computer performance may be at risk.
If you keep a large number of tabs and windows open in Firefox, sometimes you want to glance at everything that's open to find the one page or Web tool you need RIGHT NOW. Enter Showcase. This long popular tab preview application is looking a bit dated, but it does what it's supposed to do--provide previews of all your tabs--very well.
Showcase provides four main ways of previewing your Firefox tab content. All of the options are available under the Firefox -> View -> Showcase menu: F12 displays all open tabs in all windows in a new Showcase window; Shift+F12 displays all open tabs in the current window; Showcase All Windows Tab (Ctrl+Alt+T by default) shows the same thumbnails as F12 (show all total tabs) in a new Firefox tab; and Showcase Tab (Ctrl+Alt+Y) mimics Shift+F12 in a dedicated Firefox tab.
There have been a lot of tab-previewing add-ons that have launched since Showcase's initial release, and competition has resulted in some nifty developments--my favorite is probably Tab Scope, which provides similar previewing functionality, within the frame of a navigational tool. For now, however, Showcase remains my go-to tab-preview extension.
Which Firefox add-ons are your favorites for managing large numbers of tabs? Tell me why I'm wrong (or right) in the comments.