First, did you make sure that the Firefox window had focus?

Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove the profiles folder.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Yes, firefox window has focus.

I did what you've suggested but this is still happening to me.

Were you able to reproduce the issue? A coworker just tested it and this happened to him too

On my v54, <Alt> W does nothing. Were you trying to open a new window? <Control> N

You have to use the combination in the fiddle I provided

Pressing ALT + w should output "combination detected" Releasing the combination the way I explained should output "combination stop"

I'm about to post result of test cases in different platforms

Test cases...

me: w7/w10 x64 Firefox Quantum 58.0.2 x64 NOT WORKING coworker 1: w10 x64 Firefox Quantum 58.0.2 x64 NOT WORKING coworker 2: ubuntu 16.4 x64 Firefox Quantum 58.0.2 x64 WORKING AS EXPECTED coworker 3: macOS High Sierra 10.13.3 Firefox Quantum 58.0.2 x64 WORKING AS EXPECTED

I called for more help.

any news on this?

As far as I can tell, Alt behaves differently because it is the key that activates the menu bar. When you tap and release Alt, the document records keydown only, and the menu bar is activated. Focus has left the document. When you tap and release Alt again, focus leaves the menu bar and returns to the document, and a keyup event is recorded.

https://jsfiddle.net/e4berr36/20/

(Note: tested in Firefox 58)

But this used to work...

Something is happening in FF for Windows. (Linux and Mac are working just fine)

mcanepa said

But this used to work...

I have a theory. When I test in the Extended Support Release of Firefox 52:

about:support => Multiprocess Windows 1/1 (Enabled by default)

press alt + W
keydown fired: Alt
keydown fired: w
combitation detected
keyup fired: w

about:support => Multiprocess Windows 0/1 (Disabled)

press alt + W
keydown fired: Alt
keydown fired: w
combitation detected
keyup fired: w
keyup fired: Alt
combitation stop

Were your previous tests with multiprocess disabled? In Firefox 58+ (possibly Firefox 57+) you can no longer disable multiprocess.

If you think the old behavior should be somehow replicated on multiprocess, if that is possible, you can file a bug report or see whether someone has done that already: https://bugzilla.mozilla.org/

Something is happening in FF for Windows. (Linux and Mac are working just fine)

I don't think Alt has the same significance on Linux and Mac applications due to their use of a persistent menu bar.

@jscher2000 maybe you have something here!

I don't know if I had multiprocess disabled, but if this is something that changed in recent versions maybe that's whats causing the issue.

I will report the bug and see what happens

Please provide the bug report number.

https://bugzilla.mozilla.org/show_bug.cgi?id=1445724

Well done.