hi, firefox 26 is a very outdated version (there has been a change in the password storage format since then) - you've downgraded there instead updated! please use the current version of firefox developer edition, which is available at https://www.mozilla.org/en-US/firefox/developer/all/

... and horror of horrors, I can't use Firebug!!!

This is just getting worse...

Thanks, I will try that.

The automatic "update" tells me that it is the latest version, 

"Nightly 26.0 Mozilla Developer Preview is up to date."

I didn't do anything, it just updated itself this afternoon.

this is quite curious then - you also have a user.js file present in your profile folder apparently, which is often a sign of adware on your system in case you haven't created it manually - please remove it as explained in the first article and run a scan with a tool like adwcleaner or malwarebytes: How to fix preferences that won't save Troubleshoot Firefox issues caused by malware

I followed your first instruction. I am now running 44.0a2 (2015-11-25).

However, I have followed the instructions to find user.js, and I don't see it there now. Can I assume that it was removed by the installation of the latest version? Or should I look further.

I am upgrading the virus checker. The one that I had, I wasn't happy with, and I foolishly left it till "tomorrow".

The strange thing is, although I am a developer, I wasn't using the developer edition, as I just hadn't had time to get into it. Yet the "upgrade" moved me to an old version of the Developer version.

A little more time now, so I will might as well get into it!

Thanks for your help.

new versions of the firefox developer edition run a separate profile (that may be why you're not seeing a user.js in the profile - and probably no bookmarks and passwords either). you can change that by toggling the "Allow Firefox Developer Edition and Firefox to run at the same time" in the firefox menu > options > general section.

Yes, found it now. And the "standard" Firefox is now available. That is where user.js is located. It contains 10 references to the firewall, and nothing else user_pref("extensions.zonealarm.tlbrSrchUrl".....

I have removed 17 bad files. One was Trojan.Agent, C:\Program Files (x86)\Windows 7 Activator\Windows7Loader.exe, The other 16 were related to these... PUP.Optional.InstallCore, C:\Users\admi\Downloads\FileZilla_3.exe, , [f4446a188cff57dfb7820b3f90713dc3], PUP.Optional.OutBrowse, C:\Users\admi\Downloads\Windows 7 Activator.zip, , [0533cfb34a41be78093dd23d51b0ed13], PUP.Optional.DownLoadAdmin, C:\Users\admi\Downloads\gimp-setup.exe, , [fc3ce59daae15adcba6a59ccc43ddb25], PUP.Optional.InstallCore, C:\Users\admi\Downloads\MediaPlayerLite-0540.exe, , [91a7bbc7523971c5d6a095f411f37a86], PUP.Optional.Olacarita, C:\Windows\Installer\51890.msi, , [3800dfa3ddae7bbbb147770bf80cdb25], PUP.Optional.Spigot, C:\Users\admi\AppData\Roaming\Mozilla\Firefox\Profiles\c8g5b47p.dev-edition-default\prefs.js, Good: (browser.startup.homepage", "https://www.malwarebytes.org/restorebrowser/), Bad: (browser.startup.homepage", "https://es.search.yahoo.com/?type=926458&fr=spigot-), ,[ec4c2959a9e2e74f7ac3c4cf5fa57c84]

Anywho, it appears to be working, and the issue appears to be resolved with your assistance. Thank you very much