>>398
Firefox 87.0 のシステム要件
https://www.mozilla.org/en-US/firefox/87.0/system-requirements/
> Windows
> Operating Systems (32-bit and 64-bit)
> 　・Windows 7
> 　・Windows 8
> 　・Windows 10
>
> Recommended Hardware
> 　・Pentium 4 or newer processor that supports SSE2
> 　・512MB of RAM / 2GB of RAM for the 64-bit version
> 　・200MB of hard drive space

Windows 7自体は問題ない
相性の話ならむしろPCのハードやドライバの方だろう