Firefox is without doubt one of the most memory-optimized browsers out there in these days. The Mozilla team has done a great job at improving the browser's memory use.
While memory usage can still climb to crazy levels if you open enough tabs or run an add-on that increases memory usage by a lot -- Adblock Plus is a great example of that -- it is fair to say that it is quite good when compared the memory requirements of other browsers such as Google Chrome.
There is still room for improvement though, especially if you have lots of tabs open at all times. While it can be comfortable, there are other ways to deal with it instead.
Firefox's own tab groups feature may work for you, as it allows you to add tabs to groups and only display one of the groups in the browser at the same time.
While that may improve manageability, it does not really affect memory use.
That's where the free add-on Tab Grenade comes into play. Tab Grenade's core feature enables you to save all open tabs of the current browser window as a tab list.
The open tabs are removed from the browser window which frees up memory in the process. You still have access to all tabs that were open, but in list form and not as tabs.
To use the extension, simply click on its icon to turn all tabs into a list. There does not seem to be a way to load the list without doing so directly which feels a bit strange.
The resource page lists the number of tabs, the date the tab list was created, and all website titles and links. You get options to open all links of a group again or to open links individually instead.
There is also an option to delete a tab from the group, and to share the listing as a web page. Doing so will create a copy on the tabgrena.de website where it is publicly available (obfuscated only by address).
The only downside of the add-on is that you cannot launch internal pages from the tab list page as they won't open at all. I have tested this with several about: pages and they would not open. It can be that other internal pages won't open as well, but I have not tested that.
A quick test with ten open pages revealed the following result: The Firefox process used 274 Megabyte of RAM with the ten pages open, and 175 Megabyte after Tab Grenade was used to turn the tabs into a list.
Tab Grenade can help you recover memory used by Firefox. It may not work in all situations, and the fact that you cannot open the resource page without invoking the add-on's functionality is a usability issue, but if you don't mind that, it can really help reduce the browser's memory usage.
A comparable extension for Google Chrome is Simple Windows Saver with the difference that it won't remove the tabs that it saves to a list automatically.