I'm going to update this post, because I have had the chance to experiment more with the Pi 4, and although I believe the original post was about the Pi 3B+, people with the Pi 4 might be finding this thread.
For a fast, lightweight browser to play video in various ways, kweb is excellent, provided you don't want to try watching 4k. kweb Minimal Kiosk Browser was developed especially for the raspberry Pi in the days when the Pi had very limited RAM and CPU/GPU power, so it has very low overheads and runs fast.
As far as I know, omxplayer (used by kweb) is still limited to 1080p. And realistically, for many of us, the difference between 1080p and 2160p is hard to tell visibly, even on a UHD TV.
However, back to Firefox vs Chromium.
I like Firefox, but Chromium on the Pi, and on all of my other Linux computers, just seems to work more smoothly. But watching videos like YouTube video can be frustrating when waiting for content to stream can be painfully frustrating (Australia's NBN is a good example).
Where kweb shines is that it installs a script called youtube-dl, and sets up a server, which downloads the video content from YouTube or similar sites into a cache, then once the cache is at the right level (I believe this is how it works) it streams it to the screen using omxplayerGUI. That means there is a small delay while kweb is setting this up, then you have beautiful high resolution video up to 1080p.
However Chromium on the Pi 4 uses VLC by default to play back the video. And my experience with that has been sketchy.
Guenter Kriedl, the creator of kweb Minimal Kiosk Browser has provided a simple script that will allow Chromium with VLC to use the youtube-dl server to play video.
The script is pasted into a new 'Bookmark Page' on the Chromium Bookmarks Bar', and Chromium is run using a command that starts the youtube-dl server in the background as well as starting Chromium. Then when you open a YouTube video, you simply pause it, and click the bookmark (Mine is just called PLAY). There is a small delay of a few seconds as youtube-dl begins the extraction of the best quality video it can find on the page, then a VLC window will open and the video will play.
Depending on how the youtube-dl server is configured, the video can open in VLC or omxplayerGUI, but that can be easily switched in the kweb settings (under Switch Server or something).
In theory this can also be done in Firefox, however my Firefox is configured for a particular task and I have not tested it. But on Chromium, the difference using this method compared to just VLC 'in' Chromium, is amazing.
If anyone is interested, leave a reply here and I will try to find the links to where I posted the method to set this up previously.
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until you fart ...