Uh-oh, closing the browser at step "b" from the blank page probably replaced your actual previous session with the useless start page.

Why didn't Firefox restore your session automatically at that time? I don't know the reason...

Since you know the location of previous.js, are there any update files that look like good candidates for restoration?

If you check your desktop, do you find an Old Firefox Data folder with a semi-randomly-named profile folder inside with a recently modified sessionstore.js, recovery.js or previous.js file?

If so, perhaps the problem was caused by a Refresh. Although the Refresh feature copies your last session to your new profile folder, it restores the default startup setting. So at that moment, unless there was an overriding instruction in a user.js file to restore your previous session, you would have needed to do it manually from the History menu.

Actually, judging from your extensive list of Extensions, a Refresh is unlikely to have gone unnoticed, so that hypothesis can be ruled out.

Is session history working normally now, i.e., it was a one-time glitch, or has it stopped working altogether?

Also make sure that you aren't using Private Browsing mode windows or tabs possibly via the Private Tab extension.

• https://addons.mozilla.org/firefox/addon/private-tab/

In case you use "Clear history when Firefox closes".

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.
• Clearing Cookies will remove all specified (selected) cookies including cookies with an allow exception that you want to keep.

Wow. Whatever weirdness caused that half-formed page completely borked my ability to restore previous tabs. I took me a bit to actually figure out how to get to my question, since that was mysteriously missing from the history (all the other tabs that I opened were in there, further deepening the mystery).

No I found the ability to go back months and find something I opened before is useful, so I rarely clear my history.

Is a refresh likely to help? And if so, do I need to back up everything? I'm going to try safe mode.

Unless you want to reinstall all your extensions, I'd save a Refresh as a last resort.

In rare cases, an update can fail to completely replace old version files with new version files in the program folder. This can lead to a range of inexplicable breakage. If no other explanation appears, you could address that using:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(1) Download a fresh installer for Firefox 39.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox (if applicable).

(3) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement after this reinstall?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Actually, entering Safe Mode seems to have solved my restore tabs issue (I think). Here are the steps I followed:

1. I went to Options and changed the option for "restore my tabs" back and forth, to "reinforce" what I wanted it to do.
2. I restarted in Safe Mode, which failed to do anything other than open the start page.
3. When I opened more sites from my history in different tabs, I restarted in Safe Mode again.
4. That seemed to work, so I restarted in Normal Mode with all my lovely extensions.
5. Which brought my restore tabs back. Now I'm digging through the previous.js to get my tabs back. There were only about 12 of them, so it's not a monumental task.

I want to thank each and every one for your replies. Even if they don't help me, maybe I can help others with the info.

A quick way to inspect a sessionstore.js file or a file in the sessionstore-backups folder is opening the file in a tab and evaluate this JavaScript code in the Web Console (Firefox/Tools > Web Developer).

eval("ss = "+document.body.innerHTML.replace("<pre>","").replace("</pre>",""));
sd='';
for (var i = 0; i < ss.windows[0].tabs.length; i++) {
 var ent = ss.windows[0].tabs[i].entries;
 var last = ent[ent.length-1];
 sd+="["+(i+1)+"]:"+"<a href="+last.url+">"+last.url+"</a><br />";
}
document.body.innerHTML=sd;