Firefox 21-31 used a database file named signons.sqlite in coordination with key3.db to store logins. Firefox 32-49 now use logins.json in coordination with key3.db to store logins.

What version do you have right now? If you are using a version newer than Firefox 31 -- Firefox 32-49 -- you could try setting up Firefox to redo the conversion from signons.sqlite to logins.json as follows:

Overview

Step 1 is to change a preference Step 2 is to exit Firefox and hide logins.json Step 3 is to try a new conversion

Toggle a preference

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste signon and pause while the list is filtered

(3) Double-click the signon.importedFromSqlite preference to switch the value from true to false (after Firefox does the conversion again, this will switch back to true on its own)

Hide the messed up logins.json file

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. This should launch a new window listing various files and folders in Windows Explorer.

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

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

Pause while Firefox finishes its cleanup, then rename logins.json to something like logins.old, or I guess you could delete it if it's really of no use.

Start Firefox to trigger a new conversion

Start Firefox back up again. Were the logins converted successfully this time?

After reverting to a backup of FF21 directory & profile (not sure what those do that get screwed), I tried to reinstall the latest, but got the same result. This tells me, that the conversion is not working properly out of the box. Will try a third time and then run above procedure, but this should definitely not be and is a sign that FF releases are not tested thoroughly. Let me ask again, if there is a safer way, updating from 21 to vX then to vY then to vZ etc? Or, if apt, apologies for the above, what am I missing?

In the end I went for ESR, which was not v45 (as ESR initially says) but v38, but at least it has the tiles of newtab not blurred (yay hurray) and the type is legible!

And...the above procedure worked, as it seems all pwds are working (so far same behavior of FF on all login pages as before...and improvements(!) yay, re redirection & multiple confirms gone e.g.).

Overall a bit jerky judging from the activity halo (the turning loading ring), and even slower (when only one tab is open) than FF21 (when it had 200+ tabs open, and loading one new), but at least all sites work (no more ssl probs or new browser needed hints or wrong display, etc).

And ESR allows loading of unsigned addons ;)

Thx Mozillians and FF Masters! (Oh, I also donated last night already...)