Environment: - Firefox v. 55 - macOS Sierra 10.12.6 - Typical browser session = 5 to 20 tabs Behavior change: When I single click a background tab to bring it forward, the tab closes 3 out of 5 times. I am careful to not click the X in the tab's right-hand corner. While there's an obvious risk of hitting that bullseye when I have 20+ tabs open (e.g., can't even see the X), the tab "crash" occurs when I have as few as 3 tabs open and am clicking right in the middle of that wide thing. I would prefer to not have to pin or protect every tab I open. And while I can easily recover a closed tab, there is data loss if I was in the middle of editing or composing something on that closed page. I checked my mouse settings in System Preferences, but there's only an option to change the double-click speed. There is no setting for single click behavior that I can find, and internet searches haven't helped. I am perplexed.