I use 1 account on Win 7, 10, and different versions of openSUSE. see screenshot On Tumbleweed I run 3 different version of Firefox and having them has recently saved me because Nightly crashed and lost 50 tabs and history, but I had things synced on version 88 and was able to send the Nightly tabs from 88 back to Nightly. Sync sees the versions as different devices.

Thanks a lot, @jonzn4SUSE! I suppose those are very illustrative examples. I'll give it a try. Best regards.

PS: by the way, when you say "was able to send the tabs from 88 back to Nightly", how did you did that?, was it in a simple action or you had to re-open all tabs in 88, bookmarked or whatever, and then re-send them to the Nightly session, or some other way? I've been in that situation, and a good trick is always welcomed to be learn.

Opened 88, then open a new window, then do ctrl+h to open the history tab, click on the drop down to switch to the Sync Tab. From here you can open all the tabs from the other devices. Then I open all the tabs from Nightly, right click on 1 tab to then select all the tabs, and send to Nightly. I opened Nightly which had only 1 tab because the browser crashed and then in a few ticks, all sent tabs started loading. My Nightly tabs restored.  ;-))