Update: If the old profile is vulnerable to malware (webssearch comes to mind), and needs to be replaced, which files (passwords, bookmarks and such) can I safely copy into the new profile?

You can copy files like these with Firefox closed to the current profile folder to recover specific data. Note that you should be cautious with copying specific SQLite database files if you previously had problems.

• bookmarks and history: places.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

See also:

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

Thanks for your reply, Cor-el. I will try that. I have been using my shared (between W7 and W10) profile for a couple of years now, but forgot how to edit profiles.ini (and now I also see installs.ini, which seems to be new), so that they both point to the same profile on my E: partition. (I like the fact that I could reboot from W7 to W10, and find exactly the same content, when I launch Firefox in W10, and continue where I left off). So, in short: how do I edit profiles.ini and installs.ini to make them point to my profile on th E:-partition?

Update 2: Copying those files you mentioned has worked, for Win7, but Sync does not work for me; I've enabled Sync on all three OS's, and all three versions list all the OS's, but I only get my passwords and bookmarks on Win7. It seems that Sync will only work, when 2 or more devices are active simultanously; in other words: I am obliged to buy a smartphone (and ditch my trusted 10-year old mobile phone) if I ever want to get any joy out of Sync. That is not an option, in the foreseeable future; I can't afford the price and especially the running-costs of a smartphone. I rarely feel the need to browse the net, or to use anti-social media, when I go outside. I was able to extract my original profiles.ini from an older backup, and it only has three lines, where the new one has 15 (!) lines of text. The new profiles.ini does have five lines pointing to my old setup, but lists it as 'profile0' and says, under the heading [General] : "StartWithLastProfile=1", and: "Version=2". The third line of both 'profiles.ini' and 'installs.ini' says: "Locked=1" - do I need to edit that, as well? Any help in this matter would be appreciated.

Sync should be working with two desktop devices.

What items are you syncing?

You can open about:sync-log via the location bar to see if there is a recent log available. You can upload a recent log if the log is too big to put in a reply.

• https://pastebin.com/

Best is to let Firefox do the work and use the Profile Manager to manage profiles. You can add or remove/hide profiles. To make Firefox use an existing not listed (copied) profile you can use the "Choose Folder" button and browse to this folder. "Choose Folder" should never be used to select a folder that has other files in it that aren't part of Firefox.

installs.ini locks a profile for a specific installation path and Firefox will refuse to use such a locked profile if the computed hash of the installation path doesn't match. Sharing a profile between 2 operating system isn't that easy and you may have to swap various files to make this work (profiles.ini and installs.ini and compatibility.ini in the involved profile folder).

Here's the link to the log on pastebin: https://pastebin.com/hGEHid72 I hope that link works, as I'm not familiar with Pastebin. The log was created today, with the latest start of Firefox. Apart from copying the files you mentioned earlier, I never touch the profiles folder, so everything in it is genuine Firefox stuff. I use Win7, Win10 and Linux Mint 19.1 - each OS has its own SSD or HDD (Linux Mint on HDD). Win7 is my default OS at boot-up. I have been sharing my profiles for Firefox and Thunderbird between Win7 and Win10 ever since Win10 was introduced, and never had any problems, untill now. (Linux Mint is too complicated to share, so there, I just left Firefox to work as a standalone program, and do not use Thunderbird). I will try to use Profile Manager, and hope that will work for me.

Update 3: Before acting on the previous post, I noticed a small yellow triangle at the Firefox-menu in Win7. When I clicked on it, it told me I was NOT logged in to Sync. Weird, coz both other instances did NOT show that, and STAY logged in. Sync now seems to work, but it's taking its time, it seems. (Should not really be a surprise: my old profile was more than 1 GB in size. Yeah, I'm a longtime user of Firefox....) I can now synchronise my current tabs across all three platforms, but I can't see my collection of logins and passwords, or ALL my bookmarks. I hope that, with time, all of that stuff will also be synchronised. So, it's too early to call the problem solved, but I thank Cor-el for his input; at least, I got my old stuff back on Win7, and I hope to see it all appearing soon on Win10 and Linux Mint. I will keep you informed - back in a week.

Final update: OK, so I can see my bookmarks in all my OS's. And I can sync all my tabs, one by one, but not exactly as they were last showing in my default OS (Win7). I need to manualy refresh them; that will update the tab, but not necessarily to the same way as it was in Win7.

On the other hand: I still can't see my passwords synchronised, so I'll have to copy the relevant files to Win10 and Linux. I did notice an option to import such data from Internet Explorer or Edge, but strangely enough, NOT from other Firefox installs.

In all, I still have mixed feelings about the latest update to Firefox and about Sync, but if it means that FF is now more stable and secure, I suppose I'll have to live with it. (Before the upgrade, a tab could just die, and would not reload, not even from a bookmark; that problem has not -so far- recurred).

Many thanks to Cor-el for his input and assistance in this matter.