With the launch of the new, global login system of Stack Exchange, using multiple SE accounts are now impractical. [Before your downvote: using multiple SE accounts are completely allowed here.]
I think it would be highly useful, if a Firefox extension would exist, which would enable to use multiple site cookie tables, and switch between them easily.
Does such a Firefox extension exist?
As we are here to recommend software, and OP explicitly asked for a Firefox extension, let me add to ᔕᖺᘎᕊ's answer:
Using profiles as suggested by ᔕᖺᘎᕊ is a good start – but to always have to go via Firefox's profile manager is not really comfortable. To make switching between profiles easier, you might wish to take a look at the Proflist Firefox extension, which lets you switch between profiles right from within the browser:
Proflist lets you switch profiles via the "hamburger menu", see bottom right (source: Mozilla; click image for larger variant)
This solution has all of the advantages of the linked answer it builds on, and adds "better usability", more comfort, and easier/faster switching :)
Instead of using a separate extension designed for this, just use Firefox's built-in solution of having separate user profiles.
You can find more info on setting this up over here at Firefox's website.
Basically, to set up, open the Run dialog (Win + R) and type firefox.exe -p (p for profile). Everything else should be fairly self-explanatory, but check the above link if something doesn't make sense.
The advantage of using separate profiles is the browser saves cookies, bookmarks, settings and everything else separately. As this is built in, it will be regularly updated and support is easy to find, and most importantly, easy to use!
Using the profile manager like others have suggested, while practical, seems a bit overkill and may be a bit clumsy, as you would have to wait for profiles to switch constantly. (Or you could run multiple browsers simultaneously, but this would get pretty heavy and cumbersome very quickly.)
There's an extension called Priv8, which allows you to run tabs in separate "sandboxes", which, as far as I can tell, are basically just separate cookie stores (and local storage, etc).
The 'sandboxes' are permanent, and you can switch between them or add new ones with the tab context menu.
The main issues with it is it doesn't support E10s or window.open(), and isn't completely stable yet. From what I've seen though, it works rather well.
I'd suggest using cookies-based approach, instead of browser-profile ones.
Because cookie solutions:
are faster and more convenient - switching profiles takes browser restart
need less setup - no need to replicate
more direct solution - profiles deal with cookie collections too, not using profiles unless explicitly needed has less layers of indirection
it scales better - as a result of previous points
There you have many options, but in terms of user-friendly Firefox addons I've currently found only:
Priv8, mentioned in this answer
browser-wide cookie switching
reviews voiced some concern about localStorage wiping by this extension
seems to provide more fine-grained control over cookie collections over other
As of Firefox 57 AKA Firefox Quantum, installing the Firefox Multi-Account Containers extension (developed by Mozilla) will allow you to use multiple accounts for Stack Exchange and other websites, with a different account per tab or window.
Under the hood, it separates website storage into tab-specific Containers. Cookies downloaded by one Container are not available to other Containers. With the Firefox Multi-Account Containers extension, you can...
- Sign in to two different accounts on the same site (for example, you could sign in to work email and home email in two different Container tabs.
- Keep different kinds of browsing far away from each other (for example, you might use one Container tab for managing your Checking Account and a different Container tab for searching for new songs by your favorite band)
- Avoid leaving social-network footprints all over the web (for example, you could use a Container tab for signing in to a social network, and use a different tab for visiting online news sites, keeping your social identity separate from tracking scripts on news sites)
After installing the Firefox Multi-Account Containers extension, click the Containers icon to edit your Containers. Change their colors, names, and icons. Long-click the new tab button to open a new Container tab.
