Blockquote Error in Firefox

I'm trying to add the ability to create blockquotes in the editor. I added 'Blockquote' to the command list for the toolbar, and everything seems to be working just fine in IE7. However, Firefox displays a dialog box saying 'Unknown command name "Blockquote"'. It comes up once when the page loads, then again anytime I try to edit content.

Here's what Firebug spits out for the error:

FCK.ToolbarSet.CurrentInstance.Commands.GetCommand(this.CommandName) has no properties
FCKToolbarButton()fckeditorcode_gec... (line 88)
D(Object Name=ctl00_mainContent_main_fckValue Status=0)fckeditorcode_gec... (line 99)
FCKEvents("OnSelectionChange", undefined)fckeditorcode_gec... (line 29)
_GetBookmark()fckeditorcode_gec... (line 47)
AttachToOnSelectionChange()fckeditorcode_gec... (line 31)
_FCK_EditingArea_OnLoad()fckeditorcode_gec... (line 31)
CreateBogusBR()fckeditorcode_gec... (line 36)
[Break on this error] var FCKToolbarButton=function(A,B,C,D,E,F,G){this.CommandName=A;this.Label=B;thi...

Any ideas about what it could be?