When you drag a URL from Firefox's location bar to an Icons-Only Task Manager, the cursor changes to have a green plus icon that makes you think it's going to work, but then when you release the mouse cursor, nothing happens. The IOTM does not accept the drag and does not create a link. This does work with a regular task manager if you drag the URL to the left side of the panel that hosts pinned applications; it makes room and creates a new Panel launcher entry. But for an IOTM, you should be able to drag it to the main part and have it show up alongside the other icons.
IOTM doesn't support document launchers or multiple launchers for the same app. It's really not part of the design scope ...
Perhaps it should? My expectations for the IOTM definitely line up more with the request than the status quo. I've often read that it implements the macOS-style Dock pattern, and the macOS Dock accepts drags of documents, URLs, and multiple launchers for the same app.
It implements the TM part of a dock though. If you want to drag a document to the *panel* (on OS X the entire panel is called the dock) you can - just not on the ITM. On OS X there's also seperate panel areas for windows and docs like that. One thing we could do is to try and forward document drops to the containment so the panel creates an icon launcher maybe ... thoughts?
That would be better than nothing, at least, but it would deny people the ability to intermix those icons with the regular apps, which could be contrary to user expectation in the case of (say) a web URL that's a link to a web app. I think we could go even farther and actually exceed the macOS Dock's usability here! The fact that the different parts of the macOS dock accept different types of icons always confused people. (I was a Mac user for 24 years and worked at Apple corporate for 7)