Make sure you are not blocking content.

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?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Do you at least see the captcha challenge?

This issue can be caused by corrupted cookies or cookies that are blocked.

• check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions"
• https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords

Clear the Cache and remove the Cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" from websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Manage Data

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

@ FredMcD

It opened in safe mode. Thanks.

Will I have to do this every time?

@ cor-el

Yes, I can see the math Captcha at login.

I checked the permissions. All show allow except "install add-ons" and "open pop up windows" which is blocked.

I cleared the all the cookies for Wordpress but that had no effect.

If it works in Safe Mode and in normal mode with extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) disabled then try to find which extension is causing the problem by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via one of these:

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

@ cor-el

Thank you. I went through the stages you suggested and it was Greasemonkey that was causing the problems. I disabled it.

Should I keep that disabled? Reinstall it?

The Greasemonkey extension runs scripts, so it is likely that you have a script installed in Greasemonkey that causes this issue. I don't know how many scripts you have and why you installed Greasemonkey, but if you do not use this extension then you can uninstall it.

Thanks, I'll keep it disabled. I would have installed it to fix an issue, but it's so long ago that I can't remember the original reason.