The movement feels nice, but the absence of UI and/or instructions makes it hard to tell what to do. I was able to reach all the fuel cans but didn't know how to end the level. After collecting all of them, I ran out of fuel and couldn't restart either.
The design is cool, but jumping is hard. It lacks some coyote time, maybe (especially on conveyor belts). Conveyors are hard to see, and the dash skill sometimes activates before the first jump. I think some sound effects when using skills should help the player track the cooldowns.
Have you tried to start Firefox on Safe Mode? For some reason, HTML5 games played on Firefox with MacOS sometimes don't work well and Safe Mode fix it. I tried Firefox with Windows10 and had no problem.