User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:51.0) Gecko/20100101 Firefox/51.0 Build ID: 20170125172332 Steps to reproduce: Right-click (that is, press and release the right mouse button) on any link. Actual results: The context menu appears momentarily, then disappears, and a new tab opens and loads the linked page. Expected results: The context menu should have appeared and remained visible, waiting for me to click on a menu item. No tab should have opened. I have been using this behavior for as long as I can remember. When I want to open a link in a new tab, I middle-click the link. But when I want to do something else with the link, I right-click the link, then move the mouse to the desired menu item, then left-click the menu item. This has always worked, until recently. Now right-clicking the link immediately opens the link in a new tab and the menu disappears. The fact that open-in-new-tab is the first menu item might be a clue.
This issue also happens with a clean profile? https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles.
Without more details we can't do anything here, so closing as incomplete.