This is a follow-up from bug 1368567. We have some really old UI migration code dating back to Firefox 14 which we can probably review and remove, for example see http://searchfox.org/mozilla-central/rev/1a0d9545b9805f50a70de703a3c04fc0d22e3839/browser/components/nsBrowserGlue.js#1729.
Given that at least on Windows we have update watersheds for other reasons (like SHA1 signing), we can maybe align around those for deciding what migrations we can drop. We at least require users to update through 43 for that reason.
The idea may be to keep migrations up to 34-35. Knowing that we force users through 43 is a nice thing. Btw, we're discussion years old versions for which it would be more useful to throw away the existing profile rather than trying to migrate it (And create tens of issues to the user).
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