Please start by making a backup of your existing session history files. Here's how:

You can open your current Firefox settings (AKA Firefox profile) folder using any one of these methods:

• type or paste about:support in the address bar and press Enter
• tap the Alt key to activate the classic menu bar, then Help > Troubleshooting Information
• "3-bar" menu button > "?" button > Troubleshooting Information

In the first table on the page, click the "Open Folder" (or "Show in Finder") button.

If Firefox will not load the Troubleshooting Information page:

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.

Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

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

• recovery.jsonlz4: 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.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update
• various .js files from Firefox 55 or earlier

After you get Firefox working again, if Firefox doesn't let you restore the earlier session automatically, we can discuss how to persuade Firefox to use one of your backup files.

Thank you! I got those files into a folder and then simply closed and reopened Firefox. The problem with the buttons has been fixed! Could you please teach me how to reinstate a session?

Could you check whether either of these are available (not grayed out):

• "3-bar" menu button > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are grayed, check either:

• "Library" toolbar button > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• (menu bar) History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)

If that doesn't help... long post would be next.

The general strategy for restoring an old session history file is:

(A) Identify the file you want to use from your backup -- this could be obvious from date/time/size, or you may need to look inside it. I have a tool to decompress and list out the contents for reference:

https://www.jeffersonscher.com/res/scrounger.html

(B) After exiting Firefox, hide the sessionstore.jsonlz4 file and the sessionstore-backups folder from your latest session

(C) Set up the old file in the profile folder as sessionstore.jsonlz4 or, for older .js files, sessionstore.js, then start Firefox and Restore Previous Session

For (B) and (C), in more detail:

(1) From inside Firefox, 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 "Open Folder" button. This should launch a Windows Explorer window listing the various files and folders in your profile.

(2) Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup -- these changes should occur:

• recovery.jsonlz4 is moved from the sessionstore-backups folder to the main level of the profile folder and renamed to sessionstore.jsonlz4
• recovery.baklz4 is deleted from sessionstore-backups

Once Firefox has stopped updating things, while viewing that main level of the profile folder:

(3) right-click > Rename the sessionstore-backups folder to sessionstore-backupsOLD so Firefox doesn't try to use those files (it will create a new one)

(4) right-click > Rename the sessionstore.jsonlz4 file to sessionstoreOLD.jsonlz4

If you see an older sessionstore.js or sessionstore.bak file, move it somewhere else or delete it.

(5) Copy the old file you want to use and paste it into the current profile folder (please don't move the original in case this takes multiple tries)

(6) right-click > Rename the old file to sessionstore.jsonlz4

If the old file does not already have "lz4" somewhere in its file extension, it may be from the Firefox 55 era. In that case, rename that file to sessionstore.js so Firefox knows it is not compressed

(7) Start Firefox and it should read the swapped in sessionstore file and restore whatever was in the substitute file; if you do not have Firefox set to restore your previous windows and tabs automatically, use Restore Previous Session.

Success?

Yes, I have all my tabs! Thanks!