Do you have a Mozilla account in Firefox 43? If you do, then login to the same account with 56.0.2 and sync the account and all your tabs and bookmarks should be there.

Regardless of what version of Firefox you are using the same user profile is used (unless you changed it).

What about the rest of your profile? settings, bookmarks, passwords . . . . .

@Warwick Hope. Doesn't seem to work. Versions 43 & 56.0.2 are on the same computer & will both start up.

@FredMcD. Bookmarks yes, Settings I think so (cookies, updates, add-ons), Passwords not saved.

Look on your desktop. Do you see a folder called; Old Firefox?

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

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.

Please update to Firefox 60.0.1, 56 is no longer supported

@FredMcD. No, no Old Firefox. Re the links: I have only one profile, which is accessible. I don't store passwords in Firefox (so no problem there).

@Tyler. Quantum lacks essential add-ons (e.g. a useful cache browser, as far as I know).

There are tons of new add-ons that use Web Extensions, I'd suggest you check out addons.mozilla.org

Regardless, the security issues in Firefox 56 are severe enough that you should update to 60, as there is no add-on worth getting infected with malware over

@Tyler. No cache browser there that I can see. I'm sure you're right about security, but I've not had problems so far in versions 1–43.

You usually need to use the Update function in Firefox to ensure that migration code can do its job and update files in the profile folder properly (watershed; nsBrowserGlue).

• https://dxr.mozilla.org/mozilla-release/source/browser/components/nsBrowserGlue.js

@cor-el. Thanks, that worked, eventually. I backed up using MozBackup 1.5.1 (released 29 March 2011), uninstalled Firefox & deleted all profiles and Firefox folders, reinstalled Firefox 43, restored from the MozBackup backup, updated (Help — About Firefox — Check for updates — Update to ...) in two steps, first to 47.0.2, then to 56.0 (64 bit).