This might be caused by an add-on. A standard diagnostic to bypass interference by extensions (and some custom settings) is to try Firefox's Safe Mode.

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode."

If the site works correctly, this points to one of your extensions or custom settings as the problem.

To further disable add-ons, you can try here:

orange Firefox button or classic Tools menu > Add-ons > Plugins category

Any change?

Yes. In safe mode there was no redirect.  :-) However, I tried disabling all of the add-ons in regular mode but that didn't work. What else might cause the issue? Thanks

Could you check the following page:

Help > Troubleshooting Information

In the second table, under Extensions, does anything still show true under Enabled? If so, those can be disabled here:

orange Firefox button or classic Tools menu > Add-ons > Extensions category

In the third table, it's rather long to skim. If you press Ctrl+f and search for clcache is anything found?

All extensions now 'false' No sign of clcache in the 3rd table.

I checked in regular mode and it doesn't redrict now. I guess the next step is to see which extension is the culprit. Right? (I think it might be the Craigslist Helper)

So the culprit is Craigslist Helper. I checked the Options for Helper but there doesn't seem to be any setting that looks like it could relate to redirects. Is there some other setting that I can change?

I don't see it on the official add-ons site. Where did you get it? Who supports it?

Craigslist Helper 0.0.23 ID: trueziinkcl@Ziink

The homepage is shown as https://www.ziinks.com I'm not sure how I got this - Craigslist or Firefox or?. It enhances the images and ad previews as well as notes. I also just noticed that the extension just switched to 'true'!

As I do some research ZInk doesn't support CL Helper anymore. The last fix was for FF 14 , thus my bug with FF 15. I found this add on very useful up until the recent bug.

One option is to try this new Craigslist Viewer that is open source. http://sourceforge.net/projects/clreader/?source=recommended

UPDATE: Trying the Viewer and it works fine, however it is a separate program that needs to load. No bugs as yet and has some handy features compared to CL Helper. Thanks to Jefferson for steering me in the right direction on my problem!

For a while, there was a project on SourceForge.net that picked up the "Craigslist Helper" project where Ziink left off. But it was removed a few months ago. It had versions for both Firefox and Chrome. I have a copy of CLHelper 0.0.0.48 FF.xpi and can share it. I've found the Chrome version is still available on Softpedia, but haven't found another source for the FF version.

Craigslist Helper was a great little plug-in. I wish someone would pick it up again.