As of today...
-- Firefox (52.0.1) when a video is clicked on Twitter, hems and haws before saying "the video can not be played";
-- IE (11.0.9600.18617) thinks the video's already been clicked once a video-bearing tweet comes into view, and "the video can not be played" is already displaying
Chrome displays, and plays, tweetclips normally.
I'd rather have FF for all the addons I've added; it would take too long, or bore me to sleep, trying to figure out how to do the addon things on a diff product.
64-bit Win7 Home Premium, custom to HP/CompaQ (CQ2014, new 2011), SP1; AMD E-300 APU w/Radeon HD, 1300MHz, 2 cores, 2 logical procs; 3GB RAM; 500GB HDD