When the browser is open, a new window should open under those circumstances.

https://support.mozilla.org/en-US/kb/firefox-already-running-not-responding

https://www.basezap.com/cant-open-firefox-because-a-copy-of-firefox-is-already-open-mac/

https://apple.stackexchange.com/questions/196771/cant-open-firefox-because-a-copy-of-firefox-is-already-open

http://kb.mozillazine.org/Firefox.exe_always_open

I assume when the browser is closed you don't get that error. Check anyway.

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh).

Now try to open a link from another application. Is the problem still there?

No problem when the browser is closed. The other links were not relevant as I'm only using a single profile, not on a Mac, and the Firefox executable doesn't remain open when I close the browser.

I will test running in Troubleshooting mode and see if the problem occurs - although under normal circumstances it doesn't happen immediately after restarting the browser.

Same problem happened straight away (to my surprise) under Troubleshooting mode

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Firefox ESR; Extended Support Release {web link}

Beta, Developer, Nightly versions https://www.mozilla.org/en-US/firefox/channel/desktop/

Install Older Version Of Firefox {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) Note: Check Both Folders

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 any profile folders.

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.

How to Start all Computers in Safe Mode; {web link} Free Online Encyclopedia

Make sure you do not start Firefox with the -no-remote command line switch.

Also make sure you haven't set the environment variable MOZ_NO_REMOTE=1

• Control Panel -> System -> Advanced -> Environment variables

If you have a MOZ_NO_REMOTE environment variable then remove this variable and do not change its value to 0 because that doesn't work (Firefox checks for its existence and not the value).

Hi All,

I have uninstalled and reinstalled and I'll give it a bit of time and see how it goes.

Cor-el - nope, didn't have the environment variable set and hadn't changed my shortcut either.

So far so good! Browser has been open since reinstalling several hours ago and the problem hasn't occurred yet.

Thanks for the help all, my computer has been running non-stop since the reinstall and the problem hasn't reoccurred.

Glad to help. Safe Surfing.