If you need to use a proprietary plugin, you should switch to the Extended Support Release of Firefox 52 instead of following the progression to Firefox 53, etc. More backstory and a key link:

As you probably have read by now, the regular release of Firefox 52 only allows one plugin, which is Flash. The Extended Support Release (ESR) of Firefox 52 will still run other old plugins that ran in Firefox 51. ESR was first developed to meet the needs of large organizations that wanted Firefox to change much more slowly, but anyone can use it. It gets security updates but no feature changes for about a year at a time.

More information on this change and ESR: Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?

Windows Users: When you get to the ESR installer list, you'll notice at least two different downloads for Windows:

• Windows installer is the standard version for Windows that runs on both 32-bit and 64-bit Windows Vista through 10 (and 32-bit XP), the best choice for most users
• Windows 64-bit installer ONLY runs the Flash and Silverlight plugins, so that is not what you're looking for. (If someone reading this only needs Silverlight and is considering switching to 64-bit, there may be some extra file cleanup to do on your system. See: How to switch from 32-bit to 64-bit Firefox)

Success?

worked on it for the last 2 hours, no luck. installed several versions of the esr...So irritated with the lack of support and pluggin crap. I have worked in IT for 37 years...this has been very frustrating...just trying to view my own web cams and maybe send a relative a link. I hate browsers...sorry to say. Will try again to install a version 2 or 3 years old to get back functionality. Could care less about the risks. Jc

I suggest posting on Foscam's forums. There are numerous threads there.

http://community.foscam.com/

Also, why not use Internet Explorer for the camera and use an up-to-date Firefox for normal web browsing? According to Foscam, there is significantly lower functionality with their Firefox plugin compared with their ActiveX control for IE.