The passage of time is a problem, but you can take a look at a couple of things.

First: back up the session history files you currently have. Here's how:

(1) To open your profile folder...

If Firefox is still running:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button.

If Firefox is closed:

Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

(2) Copy out session history files

In your profile folder, scroll down and double-click into the sessionstore-backups folder. Save all files here to a safe location, such as your Documents folder. If not too much time has passed, we may be able to use them to recover your lost tabs.

(3) What files did you find?

The kinds of files you may find among your sessionstore files are:

• recovery.js: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.bak: a backup copy of recovery.js
• previous.js: the windows and tabs in your last Firefox session
• upgrade.js-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

Most likely, the recovery/previous are too new and one of the upgrade files would be the most promising. Are any from around the right time, or just before?

Note: By default, Windows hides the .js extension. To ensure that you are looking at the files I mentioned, you may want to turn off that feature. This article has the steps: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

I'll post a pic of the folder in question, then proceed from there.

BTW, thanks for your detailed explanation, it's the clearest I've seen to now.

Is March 10th a useful date to try to explore, or is that too old now?

Another potential source of more recent copies would be a Windows restore point. I DO NOT RECOMMEND using a full restore as you would probably lose a lot of data. However, there is a third party utility you can use to look inside restore points:

http://nicbedford.co.uk/software/systemrestoreexplorer/

You can see from your screenshot the path that the files would be on in the restore point. However, possibly you'll need to set Windows to show hidden files and folder:

https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

If you find good candidates, please copy them out to a safe location rather than overwriting the original destination.

Actually March 10 would be a good date to use, as I can recoup whatever lost tabs I had from that point onward fairly easily.

Once you have selected the file you want to try -- let's say the old previous.js from March 10th -- there are two options. Sorry this isn't easier...

"Roll back" Firefox to an earlier session history file.

The way you do this is to shut down Firefox, hide all its existing session history, then copy the old file into your profile folder and name it sessionstore.js so Firefox uses that at startup. Users say it doesn't always work, but it's the only current way to feed an old session to Firefox.

If "session state" is important -- the cookies in effect at that time -- then this is your best option.

The detailed steps for the "swap" were in this post: https://support.mozilla.org/questions/1144076#answer-929701

Extract the URLs from the old file and click each link to load it manually

This is a better approach when you need to examine a number of files and you don't want to lose your current session.

One-time Setup:

(0) Install the "Session Extractor" bookmarklet (see the instructions in the top bar of the page for how to install it):

https://www.jeffersonscher.com/res/sumomarklets.html#SessExtr

To Extract the URLs:

In the folder where you copied your session history files for safekeeping:

(1) Create a copy of the session history file you want to mine for URLs and rename it with a .json extension.

For example, right-click the upgrade.js... file, choose Copy, then right-click a blank area of the list and choose Paste.

Note: By default, Windows hides the .js extension. This is all easier if you can see it. http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

Right-click > Rename the upgrade - Copy.js... file to upgrade.json (or upgrade.txt if you prefer).

(2) Open the upgrade.json file in a Firefox tab. Either drag the renamed file and drop it on an existing page to load the json/txt file in its place, or right-click the file and choose Open With and use Firefox.

(3) In the tab displaying the session history file, click the bookmarklet button to run the script. This should generate a new page listing the URLs of each open tab from the file. You can select and copy this list and paste it somewhere for safekeeping, and/or you can just use the links now.

Unfortunately, if you want to save that page "as is" (as an HTML page), there's an extra step: press Ctrl+U to launch the "view source" page, and save that as a .htm or .html file. For some reason, saving the original page gives you the original session file instead of the formatted list of links.