Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

The problem persists in Safe Mode.

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 ++++++++++++++++++++++++++++ Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder if there is one.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. If all goes well, remove the OLD folders when you are done. +++++++++++++++++++++++++++ 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

Create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I created a new profile and the problem persists.

Hi   !
Let's give this a try :
3-bar menu => Options => Advanced => General tab Under   'Browsing' Uncheck :   'Use hardware acceleration when available'

Ant difference   ?

Use hardware acceleration unchecked - problem still persists.

Stab in the dark,   but if you're using a mouse : Check it's settings or :   unplug it, wait a while and plug it back in. (or try a different mouse)

I doubt my mouse is causing the problem. Nothing changed on my system, except for the Firefox update. My mouse is bluetooth and runs off the internal bluetooth system.

You haven't mentioned trying FredMcD's suggestions   ......  (?)

As a last resort,   but   last   resort only :

Refresh Firefox - reset add-ons and settings

Are you using Adobe Flash Player 26 Beta version   ?

Your system details show :   'Shockwave Flash 26.0 d0 '

I am not running the beta version of Flash. I only use the current release versions and update them through System Preferences. I have not tried the Refresh Firefox option, as I do not have the time right now to redo all of my add-ons.

Let's remove flash and install the current.

Flash: Grab the uninstaller from here:

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html Uninstall Flash Player | Windows

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html Uninstall Flash Player | Mac

Then reinstall the latest version.

Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version: 25.0.0.171

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

WakkoWarner said

I am not running the beta version of Flash. I only use the current release versions and update them through System Preferences. I have not tried the Refresh Firefox option, as I do not have the time right now to redo all of my add-ons.

Do the refresh as a last resort only   -   let's see if installing the regular version of Adobe Flash Player does the trick first. Just curious :   how did you wind up with that Shockwave 26.0 d0   ?

Uninstalling and reinstalling Flash did not work - the problem still persists. I ended up with that version through the Adobe updater. And everything was working fine with that version of Flash before the Firefox update.

I just did the refresh and the problem persisted with no extensions installed.

Bummer   ! There's only one thing you haven't tried yet :
FredMcD suggested   "Let’s do a full clean re-install;"

Are that CSS menus or JavaScript menus?

Do they open on hover or do you need to click with the mouse to open the drop-down menus?

Can you try a different mouse in case there is a problem with the mouse you currently use?

The menus that were not working are the ones you have to click on. The hover type work fine. I decided to go ahead and try a USB mouse. The problem was not there. So, I removed my regular mouse from the system (deleted it), then paired it again.

So far it is working properly. I hate problems like this that make absolutely no sense. Thanks for all the input.

You're welcome and glad that you managed to fix it.