User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0

Steps to reproduce:

I had about 30 Firefox windows open, with between 4 and 20 tabs open in each. Probably 200 tabs in total.
Firefox told me it had downloaded an updated version (63.0.3). I told it to update and restart.
It restarted. I then spent some time working on another computer to allow Firefox time to reload its windows and tabs.

Actual results:

When I returned to Firefox (a few hours later), I found that it had successfully loaded perhaps 3 of the 30 or so windows with their previous tabs. The other Firefox windows were created, but each contained a single "New Tab" tab with a blank URL in it. There was no indication of what URL that tab was supposed to have contained, or what other tabs had been previously opened in that window.

Note that "Options->General->Startup->Restore Previous Session" was enabled before this all happened, and is still enabled. Note also that no "Restore Session" option is available in the History menu (apparently loading those blank tabs caused Firefox to think it had restored the session correctly already). The information about what tabs were loaded seems to have been lost completely.

When I clicked on one of the few windows whose tabs HAD been restored successfully, I got a little popup window telling me about the new features in this version of Firefox. It is possible that display of this (modal?) window during session restore somehow prevented the rest of my tabs from being restored correctly. However, clicking through the 3 phases of this popup so that it disappeared did not seem to improve anything.

Those lost tabs took me months to accumulate, and I may not ever be able to reconstruct all of them. Even trying to reconstruct as many as possible of them from memory will take days. The last time something like this happened, about a year ago, I tried to dig through Firefox's session history folder to restore tabs, and was unsuccessful (saved session information no longer existed), and I am not expecting better this time.

I am Not Happy, particularly since, on looking through the Firefox database, I see that people have been complaining about this sort of problem (or at least a similar problem, although the blank tabs that I got this time seem to perhaps indicate a new and different problem) for at least 8 years (see bug #637148) and still nothing has apparently been done about it.

I have had this problem, or similar problems, happen to me at least five or six times in the past 10 years or so. Each time, I have lost information that took me DAYS to try to reconstruct. This is not a small problem, and repeated instances of this are making me seriously consider permanently switching to Chrome.

For much of the past 10 years, I used the "Session Manager" plugin to protect myself against this problem, but Firefox's recent API changes within the past few years made that plugin not work anymore, so now I am stuck again with Firefox's default behavior, and unfortunately Firefox's default behavior is apparently to occasionally lose massive amounts of session data on restart, with no warning and no recourse.

Expected results:

The Firefox session restore mechanisms should be made VERY ROBUST and should restore Firefox to exactly the state it had before a restart, without losing ANY tabs or windows. By "robust" I mean that there should be no weird race conditions when restarting (especially with regards to popups or spontaneous page loads from Firefox or its plugins). There should be no attempts to replace the current session's contents with blank tabs or with a new, empty session or a session that consists solely of a "What's new in this version of Firefox" tab, or any other sort of content. If new content is to be added, it should be robustly added IN ADDITION TO any existing prior session content and should not replace prior session content, prevent prior session content from being loaded, or cause prior session content to be deleted or ignored.

Firefox sometimes gets this right, but it sometimes gets it very, very wrong.

Firefox should NEVER, EVER, EVER LOSE INFORMATION OR TABS ON RESTART and RESTARTING SHOULD NEVER HAVE THE EFFECT OF CLOSING ANY TABS OR FORGETTING THE URLS OF WHAT TABS WERE PREVIOUSLY OPENED. The ONLY way that tabs should be removed from Firefox is if the user explicitly closes them (via the "X" button for instance) or explicitly selects in Preferences that session data should not be restored on restart (which I have NOT done). In all other cases, specifically including restart (i.e. for upgrades) and crash/machine restart (e.g. power failure) Firefox should use an "add only" model of tab management that never loses tabs.

Also, the "Restore Previous Session" option should remain (with its data) until the user clears it, and it should especially not assume that a previously saved session containing hundreds of tabs should be replaced with a new one if that new one only consists of a few tabs (such as a "what's new in this version of Firefox" tab, or a "What's new in this version of the NoScript plugin" tab, etc.). If, for whatever reason, Firefox did not restore my tabs properly, I should be able to try the restore again (possibly with intervening restarts) until it does.

Please give this some serious attention! It has gotten to the point that I am afraid to update Firefox anymore because I have repeatedly lost massive amounts of work from doing so. That results in me running older insecure Firefox builds for extended periods of time, risking security bugs. Manually bookmarking sets of tabs is not a practical solution to this problem, since it is easy to forget (especially when power failure could occur unexpectedly).

Please make Firefox robust with regards to keeping tabs across restarts. I don't really want to switch to Chrome, but I am going to have to if this continues. Eight years is way too long to wait for a data-destroying problem like this to be fixed.

I just had a similar (but not quite the same) problem on Nightly 67, I think it was 2019-03-11.

Twice in a row closing and opening the browser, once manually and once for updates firefox forgot what content was in the current tab for each window, the other tabs in those windows were fine.

Unfortunately, the current UX may mean that you loose data if you're not careful. The tracking bug 1330638 contains a large number of potential UX improvements and are waiting for engineers to take be implemented. At the moment we don't have any available, unfortunately.