I made a video queue system for Floatplane. To use this you need to have the browser extension Tampermonkey. Once you've installed the extension, create a new userscript (the plus icon) and copy and paste the code from the 2 files in this repo. Every file needs a new userscript so you'll be making 2 scripts in total.
The userscript adds a button to each video which you can click to add it to the queue. You need to manually start the first video but from what I found, all video's after that start automatically.
This script was made fairly quickly so I did not thoroughly test it. So far, everything works fine for me. If you have any issues or bugs feel free to open an issue and I'll take a look at it.