User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:62.0) Gecko/20100101 Firefox/62.0 Steps to reproduce: On Mac 10.11.6: 1) Open a long, scroll-able PDF in one tab 2) Open up a long, scroll-able webpage in a separate tab 3) Long scroll down in the PDF tab, and then immediately Command-Shift-] to switch tabs OR 4) Long scroll down in the webpage and then immediately Command-Shift-] to switch to the PDF tab Actual results: For step 3) the webpage continues the smooth scroll from a previous tab and scrolls down For step 4) the webpage continues the smooth scroll and also retains the Command key and causes the PDF to zoom out. Expected results: Switching between tabs should terminate smooth scrolling.

I have reproduced your issue on MacBook with Mac OS X 10.13.6 as follows: 1. Download a PDF and open it in tab 1 of the browser. 2. Open a relatively long page in tab 2 of the browser. 3. Scroll tab 1 downwards and immediately press "control"+"tab" to switch tabs. Actual: The tab is switched, but the scroll action is continued from the PDF tab, on the webpage tab. Expected: The tab switch would be made, but the scroll should not be continued on the other tab. 4. Scroll tab 2 downwards and immediately press "control"+"tab" to switch tabs. Actual: The tab is switched to PDF, but it is also zoomed out. Expected: The tab should be switched, but the zooming action should not be made. Note: This issue occurs on all browser versions: Nightly v64.0a1, Beta 63.0b14, Release 62.0.3.

This is a long-standing issue that unfortunately we haven't had the resources to fix yet.