I spent too many hours losing hair over this until I found this thread, so thank you all so much.
I was playing around with different types of exports and found that Edge and Chrome worked well when Iframes(thanks RomanHoefner) were included, but Firefox only started running smooth when I also included B frames in my video export.
These are the settings that gave me good results from a Shotcut export.
I believe GOP is the I frame setting in Shotcut, but nudging B frames to 3 fixed my issues on Firefox.
Hopefully someone sees this and comes up with an even better solution.