Updated: August 14, 2020
If there's one thing that has value when it comes to browsing - it's the user's browser profile. Over time, we accumulate a lot of data, be it extensions, bookmarks, UI customization, and whatnot. And then, occasionally, you may buy a new computer, or set up a new operating system, and you want to port an existing Firefox profile over, so you don't lose years worth of browsing habits and information.
You can do this via Firefox sync - but a method that has existed and work reliably before any online stuff is the simple copy & paste. Just chuck the old Firefox profile into the Firefox directory, and Bob's your uncle. Well, not anymore, it seemeth. Recently, I've encountered a problem, which made it impossible to reuse an old profile. I'd get a window that reads: "You've launched an older version of Firefox ... Using an older version of Firefox can corrupt bookmarks and browsing ..." This is nonsense, so we need to fix it.
Problem in more detail
This is actually flagged as a bug on Firefox (and Thunderbird) bugzilla. And indeed it is, because it assumes that any reuse of existing profiles constitutes conflict. Because, in my setup, I was moving a working, actively used profile from one Linux instance to another - with the exact same underlying system base, exact same version of Firefox (76 at the time of writing), and maybe an hour of usage apart between the two instances.
There's nothing old and corrupt here. But the Firefox Profile Manager only allows you to create a new profile or quit. Losing tons of valuable data is not an option. This could be a valid warning if someone uses an old, untouched pre-Firefox 67 profile, but not otherwise.
Luckily, the fix for this bug is simple: launch the Firefox Profile Manager with an environment variable or use a runtime flag that overrides the default behavior, and allows the reuse of profiles. Indeed, there are several ways to achieve this. For example, on Linux, you can do this:
MOZ_ALLOW_DOWNGRADE=1 firefox -P
Or maybe on Windows, change the browser shortcut to:
And then, everything works. Indeed, my profile loaded perfectly, including the dozen extensions I was using, except one - Plasma integration. Indeed, in this particular case, going from the Plasma desktop environment to Gnome meant this extension has no meaning, but that's perfectly fine. Everything else, cushty!
There we go. An annoying no-issue problem resolved. Sure, in some cases, using an old profile can cause problems, but then, techies know what they're doing, so this is nothing more than an unnecessary annoyance. Because ordinary folks won't be doing any copy & paste magic, and those who know things know the risks as well, so why put a useless obstacle in their way? I dread to think of the situation where Firefox profile import becomes impossible in an offline manner, AKA copy & paste. I've got a bunch of profiles that I've been using for 10-15 years now, and losing them means ... well, just using a different browser then.
Well, that's it. A short guide, loaded with emotions. P.S. I've also encountered the exact same problem in Thunderbird, and the fix is identical, so if you're also trying to reuse tons of old mail data, this may help you. I don't mind the attempt to make the browser more robust, but the checks need to be robust too - or elegant. The warning popup about the reuse only needs one extra button - Continue - to make everything perfect. And we're done for today.