In approximately March, 2017, Windows XP and Vista users will automatically be moved to the Firefox Extended Support Release (ESR).
Firefox is one of the few browsers that continues to support Windows XP and Vista, and we expect to continue to provide security updates for users until September 2017. Users do not need to take additional action to receive those updates. In mid-2017, user numbers on Windows XP and Vista will be reassessed and a final support end date will be announced.
In the meantime, we strongly encourage our users to upgrade to a version of Windows that is supported by Microsoft. Unsupported operating systems receive no security updates, have known exploits, and are dangerous for you to use. For planning purposes, enterprises using Firefox should consider September 2017 as the support end date for Windows XP and Vista
https://blog.mozilla.org/futurereleases/2016/12/23/firefox-support-for-xp-and-vista/
I would backup all your bookmarks to a file, and reinstall Firefox from here:
https://download.mozilla.org/?product=firefox-52.3.0esr-SSL&os=win&lang=en-US
You "export" your bookmarks to an HTML file, then you can "import" them back into your newest FF browser.
ETA: Forget what I typed about exporting and importing (not available in FF). Just do the Sync thing (easy to do, just follow directions). Once you have your account set up and all synced (your passwords and all your bookmarks), then you can install your new ESR copy.
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