I don't USE a proxy server!

[Dreamstalker]

Every so often, Firefox will give me an error on opening: "Firefox is configured to use a proxy server that is refusing connections". I traced the initial problem (started about a week ago) to some malware that I was able to nuke...but the issue persists and I can find no traces of said malware. It doesn't happen every time I start FF up, which is strange. I can fix it by going into the proxy settings and selecting "No Proxy"; the 'default' seems to be autodetect which it never did before.

Tech specs:
--Firefox 23.0.1 on a Win7 Home Premium machine
--Comcast internet with a Linksys router connected

The newest piece of hardware in the mix is a networked printer, which my mom's laptop will spaz out on every so often (IP address conflict which started at the same time as the desktop proxy problem and also doesn't always happen). I'm tempted to just reinstall the printer completely and see if that helps anything. Dunno what that would have to do with the nonexistent proxy though.

Desktop: every so often thinks I have a proxy server
2 laptops and a tablet: no obvious interwebs issue except can be really slow at times

[Fire_on_High]

Honestly sounds like you still may have some malware. Checked with MBAM?

[Dreamstalker]

MBAM has nothing, Spybot has nothing, and a Comodo scan also comes up clean. I'm led to think that there's something hinky on Comcast's end, but I have no proof of that (also it never happens while I'm connected, and not all the time...).

[TheSHAD0W]

You may still have a proxy server configured in the computer's environment somewhere. As a quick fix, there should be a setting in Firefox that tells it to ignore the system's proxy settings. It's still something you should find and fix, as it does mean your traffic is going through an attacker's proxy.

[Dreamstalker]

I'm finding no malware on here that I know of. Would Wireshark be able to detect a proxy server? (I just used whatismyipaddress and lagado and they couldn't find a proxy)

Comcast does seem to use a transparent proxy, but I've never had problems before now.

[Dreamstalker]

Further research shows that the printer may be the culprit; every so often it gets assigned the same IP as either the laptop or the router (if it's masquerading as the router, Firefox thinks it's using a proxy because it can't get out). Disabling DHCP on the printer should fix that.