very useful for multitasking
tip for new users: you can change the shortcut in extensions settings "Manage Extension Shortcuts"
I have it set to alt+w since that is what I was used to

Unfortunately it works only within single virtual space (or whatewer it's called in Windows using Win+Tab). It doesn't switch tabs located in different spaces.
Last Tab extension can do that thing:
https://addons.mozilla.org/en/firefox/addon/last-tab/

the developper should add in the description that we can also switch to the previous active tab by clicking on the icon add-on (it is the main use i have of this add-on).

and i'd like to answser to an other comment about the shortcut modification:
you can change it easily with firefox here is how to do https://support.mozilla.org/en-US/kb/manage-extension-shortcuts-firefox link found thanks to this page https://addons.mozilla.org/fr/firefox/addon/active-tab-history/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search

need changing default Alt+x to Alt+q or other combination. thank you

Tut genau das, was es soll. Habe schon länger nach so einem Add-on gesucht.

At last an extension that can go to previously opened tab!
Ctrl + Tab doesn't work on My Linux FireFox 52 (it just go to next tab), so this plugin made my day.
Thanks!

perfect
عالی بود

Great addons, but can I just normal switch tab in active window ?

... and like all time, it should be great to have the option to choose our shortcut... but yes, making more time to dev. Thx a lot ;;) !

You can already do this in Firefox, just go to options and turn on "Ctrl+Tab cycles through tabs in recently used order". It also shows tab previews of your most recently used tabs.