What versions of Firefox is this about, i.e. what version last used this profile and with what version would you like to use this profile ? Did you accidentally use this profile with a newer version ?

You posted using Firefox 66.0.5 on Windows 8.1. Note that forcing to downgrade a profile is not advised especially if you go from current release to an older version.

Firefox 67 and newer use a dedicated profile for each Firefox installation and lock this profile to prevent other Firefox installations from using this profile.

• https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation

I know what you wrote, I have been using netscape firefox for some 25 years.

I had some problem in locked profile method so I searched and found the last version before that locking was forced, and I installed 66.0.5 on w8.1 and w10 and it is working.

You yourself had suggested that compatibility.ini method and it worked. You wrote that there are some other changes that also need to be done, so I was requesting you to elaborate upon that. -- blabbering starts now -

If downgrading is so difficult, almost impossible, then ff had not right to force-upgrade me to newer version and newer method. You guys spoiled my setup and now not allowing me to downgrade.

Your installation program should have warned user about such a drastic change before force-upgrading.

We trusted ff for 25 years and you guys ditched your users.

Now, whenever close ff, after some time there is an error message that prompts me to restart/ quit. I am tolerating living with it, as I don't know its solution, Could be the access of some ff setup files got restricted so when it writes to those files, it aborts without writing.

The problem I was facing with locked profile was - whenever I was taking backup with freefilesync, probably that was changing some access of some ff installation file, so after back up whenever I started ff, it started as a blank new profile and restored things partly from ff sync, and sync doesn't store settings/ changes of my addons, so every time I was loosing all my 20 years of settings whenever I took backup. I searched, and found no solution to that, so I blamed it on version-wise locked profile and downgraded to before that destructive change.

I have been using the same single ff profile for 20 years, except whatever was forced-changed by you guys secretively, like this one.

Please elaborate the full fail-safe procedure for dowgrading to a previous ff version.

Thanks.

Please elaborate the full fail-safe procedure for dowgrading to a previous ff version.

Unfortunately there is not a convenient reference for file format changes made between Firefox versions, so there is not a ready checklist of what files you would need to discard to make an upgraded profile run without problems in a specific older version of Firefox. Typically, though, if Firefox can't understand the format of a file, it will discard it as corrupted and create a new one. In the case of the Places database, the new one should be built using the latest bookmark backup.

Similar story with preference changes, no convenient reference. Any preference Firefox doesn't understand will be ignored and have no effect; I don't know whether there would even be any errors/warnings. However, I certainly can't account for all of the possible scenarios.

You are the expert on what works and what doesn't with the relevant versions and your data because you're the one performing the experiment with your profile.

Now, whenever close ff, after some time there is an error message that prompts me to restart/ quit.

Is that the Mozilla Crash Reporter? If you submitted crash reports, you can view them by following the link in on the about:crashes page. However, it sounds like they will just be a shutdown hang => timeout kill, and not anything actionable.

You could tweak what Firefox does at shutdown to see whether it is linked to the problem. In particular, if you have Firefox set to clear history when it closes, you could omit certain categories of data to see whether those are related to the failure to complete a normal shutdown.