User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:54.0) Gecko/20100101 Firefox/54.0 Build ID: 20170628075643 Steps to reproduce: Sometimes, the middle-click action to initiate scrolling stops working for a specific tab. I can no longer use the middle mouse button to start scrolling on that tab, even when I navigate to a different website. Other tabs are not affected and can still be scrolled by middle-clicking. It happens rather rarely, maybe once per browsing session or less. This started happening not too long ago, maybe with Firefox 54 or even 53. I'm running the 64-bit version on Windows 7.
I think I got a repro case now, although it seems to depend a bit on timing so it's not a 100% repro: I can say now that this typically starts happening when I middle-click the page after I started navigating to a new page. So I click a link on the current page, then I press the middle mouse button, I see the scrolling icon, the new site loads and I cannot scroll anymore. The easiest way to reproduce this is probably with a slow website or internet connection.