Why do tabs and windows sometimes ignore the keyboard shortcut (CMD+W)? I keep having to go to File Close Tab

Question

Tabs in Firefox seem to ignore my commands sometimes, seemingly on random. Maybe more so when the tab contains a video, although I could be mistaken.
I'm using a 13-inch, Mid-2009 Macbook Pro running OSX 10.10.2 (the latest build.), but this also happens on my stepdad's 17-inch, Early-2008 Macbook Pro as well.

Answer 1

I think the most common reason would be a plugin intercepting the keyboard. This can happen if you use the controls on a Flash video, for example.
''This is copy/pasted from an old post:''
Over the past couple of years, users have noticed that the Flash player on YouTube was particularly "sticky" or tenacious when it came to capturing keyboard focus.
Last time I looked into this, about a year ago, I posted this summary of previously mentioned workarounds:
(1) Full screen the video and then switch back
press f then Esc - focus will move back to the page
(2) Switch to another application window and then back {WINDOWS}
press Alt+tab then Alt+tab - focus will move back to the page
(3) Click on a different tab then click back - focus will move back to the page
Some users also reported that the YouTube player behaves normally (i.e., clicking the page regains control) if you disable the Flash Player plugin's protected mode feature. There is now a hidden preference for this:
(A) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful.
(B) In the search box above the list, type or paste '''flash''' and pause while the list is filtered
(C) Double-click the '''dom.ipc.plugins.flash.disable-protected-mode''' preference to switch its value from false to true.
This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.