I'm on Debian Wheezy stable with Gnome 3.4 and have several default extensions, which on the 'Installed Extensions' page at https://extensions.gnome.org/local/ (using Iceweasel's Gnome Shell Integration plugin), are not able to be removed like the ones I have added from extensions.gnome.org myself.
These include (for me anyway - and this may not be the complete list as I probably have one or two of them enabled and thus no quick way to know all of them):
Alternative Status Menu
Auto Move Windows
Gajim IM integration
Places Status Indicator
Inspecting the HTML of the Installed Extensions page, I see no way to hack it via Firefox's developer tool (at least with my limited HTML knowledge and only from a quick look) to somehow make those extensions uninstallable via that page.
Is it as simple as deleting (e.g. for the first one) the folder
/firstname.lastname@example.org, then restarting Gnome Shell and it'll be removed from the list?
I want to do it correctly, without introducing problems later.