Hitting the 3 bars at top right to open configuration. Then hit History and then hit Restore Previous Session. (I don't usually have much luck with it but sometimes it works. (Or maybe try Restore Closed Tabs-FIRST!--if you have that option)

For future you might try an extension like this one: https://addons.mozilla.org/en-US/firefox/addon/undo-closed-tabs-button/ and in its Options go ahead and set the number of tabs to remember to something higher than the 10 which is the default.

but "Restore Previous Session" is grayed out.

Just the other day, I sent myself an email with links to most of my tabs, so I can restore them, it's just a P.I.T.A. I thought there might be an easy way, but no.

I'll try your link so it doesn't happen again, hopefully.

If you keep a lot of the same tabs open you should be able to bookmark them For instance, you have 3 tabs open then you right-click on one tab and select Bookmark All Tabs. You can create a new folder and then choose to put it on the Bookmarks Toolbar so you have easy access. When you open that folder you will have the option to Open All in Tabs at the bottom of the list.

probably will work better than what I do now. Thanks so much!

One last extension that I use is Pocket (formerly Read it Later). It gives you the option to "bookmark" things to the Pocket list. If you sign up for the free account you can export the list to a file on your computer for backup.

It is a pretty good extension but if you do as I do and just start saving everything for later then the list can get unwieldy. I use it basically to save pages I want to read but don't want them to get lost in my way too many bookmarks. Might be worth a quick look.

https://addons.mozilla.org/en-US/firefox/addon/read-it-later/

I have never B clicked on the tabs before, tried it and found all sorts of helpful options.
Just updated to newer FF browser a bit ago, still exploring.
Thanks!

I use the Nightly build so I see the changes a few months before the new versions are released (I am on ver. 33). I can't say I really like the Chrome like Options bars on the far right--something about this options panel has never displayed correctly for me. But I still prefer FF to Chrome.

One of the things that I always forget about tabs is that now you can reorder your tabs by dragging them around. It is useful when you want to close a bunch of tabs. Just drag the ones you want to keep open over to the left and when finished go to the right most tab which you want to keep open then rightclick it and select to close tabs to the right. I use that fairly often.