On that machine, I'm surprised it even runs at all. That is extremely old. The GT 1030 is okay (at least for web games) but 4GB of DDR2 memory is going to be a big issue.
I guess for simple 2D games you should be okay, with the right optimization, but it will be cutting it pretty close with such old hardware. The CPU came out almost 14 years ago.
And I think 4GB of memory (at 400Mhz) doesn't even meet the requirements to run Windows 10. I mean, Microsoft says 2GB, but that is like to launch Word, not run games.
In terms of your Firefox issue, I think that is on Mozilla. I know on Linux, WebGL is pretty slow unless you run under Wayland. From my tests, Windows should be okay, but I can at least confirm that Firefox has the worst WebGL implementation in general.