You are better of installing the old, 28.0 version. The new one is buggy as hell. ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/28.0/win32/en-US/Firefox%20Setup%2028.0.exe

Like EvanDark1 said, install the older version of Firefox, log into your old Sync account, then update Firefox and it will keep your old account logged in. I just did this and it works for me.

EDIT: I meant to also add that the work-around that I found for a pseudo-migration would be after recovering your old account in the old version of Firefox, you will want to get the new Sync account and then sync all of the old stuff you had.

Once a Sync account is upgraded to the new version of Sync it won't work with the older version of Sync.

Try Palemoon Browser its the same as Firefox but 64 bit and with Duckandgo search engine. Another fix which may require older Firefox is copy and paste the json from your bookmark backups. Firefox keeps a backup of your bookmarks on the PC. Or you can just browse to the json file you want(they are marked by date of backup) and restore. Here is the link for restore instructions. Note: you can use this instructions to migrate your desktop to Palemoon too. https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them?esab=a&s=restore+bookmark+from+backup&r=0&as=s#w_restoring-from-backups

Here is the link where json files are located on Win8.1, (Win7 and XP is has it within AppData) (Mac is common sense Linux in the Library) C:\Users\'user'\AppData\Roaming\Mozilla\Firefox\Profiles\5eczdook.default\bookmarkbackups

Palemoon is the same: C:\Users\'user'\AppData\Roaming\Moonchild Productions\Pale Moon\Profiles\e6rx8hnh.default\bookmarkbackups

Just remember the 'user' is the name you are logged on with.