Thanks for letting me know!
I actually tested the game on Firefox on a bunch of computers and didn't find any issue, so I'll look into this.
There's currently a memory issue that causes low fps on the browser, but that only happens after a while, after enough objects had spawned. I already fixed that but I am unable to upload the new version until the jam is over.
I wonder - was the game like that as soon as it loaded, or did it take a while?
I saw your post about chrome - please make sure hardware acceleration is not turned off if it runs slow over there.