Kmeleon is one of the lightest browsers there is and it looks a lot like Firefox in some aspects.
You can’t load any of the Firefox extensions but it has its own options for ad-blockers, pop-up blockers, themes, and a lot more.
You only need 256 MB of memory to run this browser and it even works on Windows 95 and XP, let alone Windows 7.

https://windowsreport.com/best-browser-windows-7/