When the multiple tab feature was first introduced by Firefox, everyone got into the habit of opening many many tabs at the same time in Firefox. Now the habit has perhaps turned into addiction and people tend to keep opening multiple tabs to the extent that the system becomes unstable. There seems to be no alternative to this behavior in the near future, but you can use the free One Tab extension for Firefox that can curb the problem of opening too many tabs and slowing down the system by turning all the open tabs into a list in a single tab. The list appears like a list of links and you can restore these tabs any time you want.
One Tab extension for Firefox can help you from your annoying habit of opening too many tabs in Firefox and uttering execrations while sporadically slamming down your fists on the table when Firefox starts to become slow to response. This extension is a no-restart extension, so it starts working as soon as you install it without the need of restarting Firefox.
You would notice a new filter or funnel like icon in the Firefox toolbar. At any time you want to convert all the open tabs into a single list in a single tab, just click on this One Tab icon. You would see that all of your tabs will disappear and a new tab will open up containing the One Tab list of all other tabs. You can click on the Restore All to restore all the tabs, or you can click on the individual links in the list to restore them as you please.
In the options for One Tab (accessible only from the One Tab list page), you can choose what to do when tabs are restored from the list (show in a new window or current window), what action to take in case of pinned tabs (turn them into list or not), whether to display One Tab at browser startup etc.
Conclusion: The One Tab extension for Firefox makes Firefox faster and stable by converting all the open tabs into a list and reducing the memory consumed. It can restore all the tabs in one go, or you can choose to restore individual tabs as you see fit.
You can get the One Tab extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/onetab/.