I occasionally use Portable Firefox on a computer that also has the normal version of Firefox installed, mostly for privacy/security reasons. The problem is that every plugin installed on the computer shows up in Portable Firefox as well. Is there a way to keep that from happening, so Portable Firefox only uses the plugins from its own plugins folder?
Yes, it is possible, however it requires editing your firefox config as listed here.
To explain what the link says (note - quotes should not be typed):
Type "about:config" in the address. Agree you'll be careful on the screen that comes up. type "plugin.scan" in the search box on that page.
Double-click on "plugin.scan.plid.all" to change the value to "false".
If desired, double click to change the values for Acrobat, Quicktime, and Windows Media player to something higher than you'll encounter for a while - something like "100.0" for each should do.