Does Firefox portable use more RAM than a normal Firefox

Well, your task manager is probably not lying, if that's what you're asking. But there are a number of factors involved.

Portable Firefox is really just Firefox configured in a special way. FirefoxPortable.exe calls the mostly-unmodified Firefox, which runs almost like normal. The launcher sets things up and cleans up after it. The launcher will hang out until firefox.exe exits, then it performs its cleanup. Together they should use a few hundred kilobytes more, tops. Nothing you should notice, nothing that should slow you down on a system with enough RAM to run the OS and other software you're running.

Portable Firefox's usual performance bottleneck is the flash drive. Run it from a hard drive and the performance will be indistinguishable (by human perception) from the regular installed Firefox. This is what I do at home, run portable apps off an external hard drive. My setup is a little different from PortableApps.com's intended setup, but it's currently the ideal setup for what I do.