Firefox has been a bit of a RAM hog, I regularly see mine close to 500MB for firefox.exe, so I wouldn't be overly concerned about an extra 20MB. I would like to see my firefox as low as 200MB RAM, but RAM is essentially there to be used, not much point having it otherwise.
There used to be another firefox entry in task manager PluginContainer.exe or something close to that. But that appears to have gone now.
I have lots of add-ons around 30 and usually have multiple tabs open and that will add to the RAM usage.