Can you start by giving us the troubleshooting information from the Firefox installation that's having problems. This is an essential step in troubleshooting.

• Click the Firefox button at the top left, then click the Help menu and select Troubleshooting Information from the sub-menu. If you don't have a Firefox button, click the Help menu at the top and select Troubleshooting Information from the menu.

Now, a new tab containing your troubleshooting information should open.

• At the top of the page, you should see a button that says "Copy text to clipboard". Click it.
• Now, go back to your forum post and click inside the reply box. Press Ctrl+V to paste all the information you copied into the forum post.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• The image "new fx menu" does not exist. > Options/Preferences> Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• The image "new fx menu" does not exist. > Options/Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

Can you try to start Firefox in Safe Mode to see if Firefox works properly with no error? You can start Firefox in Safe Mode below:

• Windows/Mac: Go to Help > Restart with add-ons disabled
• Linux: Run firefox -safe-mode in the Terminal/Konsole

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

Can you reach the Subject field via the Tab key?

What element gets selected if you right-click the Subject input field and select Inspect Element to open the Inspector?

• https://developer.mozilla.org/Tools/Page_Inspector