Go to Tools > Extensions.

Note there will probably be several "xxx is incompatible with iceweasel
6.02" add-ons there.

Note I cannot give you the exact text because copying it with the mouse
is not allowed. BUG.

Note I cannot remove them because each does not have a Remove button
because the designers of iceweasel forgot to put one there for disabled
packages. BUG.

Hence one must wait for add-on maintainer to update their add-on before
we are allowed to Remove them. How ridiculous. Now they just accrue for
life.

Wait, some of them do, some of them don't. Even though they are all
marked "incompatible with iceweasel 6.02". Here, here's a screenshot:

Given the version numbers available, and what is in the debian archive,
i suspect that you have the xul-ext-greasemonkey and xul-ext-firebug
packages installed system-wide.

Since system-wide packages cannot be installed or removed by a
non-privileged user, you do not get a "remove" button in the extension
manager UI (which is running non-privileged -- i'm assuming you're not
running iceweasel as root).

system-wide extensions usually only get a "disable" button instead of a
"remove" button; but since they're automatically disabled due to version
incompatibilities, that button wouldn't do anything anyway, so it seems
reasonable that iceweasel would hide it.

Unless you are sure that you don't have either of xul-ext-greasemonkey
or xul-ext-firebug installed as debian packages, i'd suggest that this
is not actually buggy behavior, so the report should be closed.

Regards,

And indeed it turns out I have them installed like you say,
xul-ext-firebug/unstable uptodate 1.7.0-1
xul-ext-greasemonkey/unstable uptodate 0.9.1-1
However not in my wildest dreams would I have figured out why some have
a remove button and some don't, even though I'm the very person who
installed them, as root and others just as me.

Therefore some kinds of "can only be removed by the system
administrator" message should pop up.

Or next to each plug-in, disabled or not, there should be a "*", with a
key at the bottom:
"*installed system-wide", so one can distinguish the ones just installed
on ~/ vs. those system wide.

You might argue that there is no such special marking for Unix programs
installed in ~/bin vs. /usr/bin, when one hits TAB in bash to show
partial matches, etc. But then I would argue that we Firefox users
shouldn't be expected to be as sharp as Unix users, etc.