Hi, sorry to hear you are having a problem; please see if this helps - How can I get the latest version of Firefox on Windows XP or Vista?.

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Actually that article is not applicable now with Firefox 49.0 and later.

Also Firefox 49.0 and later no longer runs on older CPU's that does not support SSE2. It was still possible to do so with 48 and earlier.

https://www.mozilla.org/firefox/49.0.1/system-requirements/.