Sure, you can apply reloading like that if you want. But I would be careful when using that as .uc.mjs because if you don't tag that script with @onlyonce - and you have like 10 windows, then Firefox will reload userChrome.css once a second for each window so potentially 10 times a second. That could be quite a lot of cpu usage for constantly invalidating and computing the browser UI styles.

Also, I don't think that function can handle the case where you use @import statements in your css.