I had the same problem & found this reply by paronigor in another thread that solved it for me:

https://support.mozilla.com/en-US/que.../810425#answer-190766

"I looked for files modified at the time I thought a similar Malware acted on my PC.

Found 3 lines added to the:

C:/users/xxxxxmyprofile/appdata/roaming/mozilla/firefox/profiles/xxxxx.default/extensionsfoxmarks@kei.com/default/preferences/prefs.js

I removed them, and I saved the prefs.js file again. They were like: user_pref("network.proxy.http", "127.0.0.1"); user_pref("network.proxy.http_port", 58505); user_pref("network.proxy.type", 1); More or less.

My suggestions: Check all *.js files in firefox/ subfolders, plugins included (in my case, as you can see, it was in foxmarks subfolder).

Took me a while, finally solved. "

@ go to Help Menu -> select "Restart with Add-ons Disabled"

Firefox will close then it will open up with just basic Firefox. You can enable the Add-ons later.

If @ solution doen't work then try following solutions and whichever setting works, keep it Permanent:

1) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option2 -> Click OK -> OK

2) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option3 -> Click OK -> OK

3) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option1 -> Click OK -> OK

Check which solution works.

Get an add on called Foxy Proxy Basic (works with all versions) from Mozilla, install it and restart. Go to the advanced connection settings tab and you will see it's been replaced with the add on. Open it and on the drop down box select "default". This will work if you don't use a proxy. Solved my problem, hope it works for all. Good luck!

Thank you SOOOOO much cloudlift. I've had this problem for so long now and finally was able to resolve it with your help. My problem wasn't with foxmarks, but personas.