Ok, finally get the Theme and Extension managers working in Firefox by adding a tarball which contains those files which Firefox creates under <prefix>/lib/firefox/ once it is run as "root" and after at least once opening the Theme and Extension managers. They are important to just exist (and do not have to be writable) to allow non-root users to actually use the Theme and Extension managers at all.

firefox/firefox.spec

@@ -43,8 +43,9 @@ Release:      20050612
 Source0:      ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/%{version}/source/firefox-%{version}-source.tar.bz2
 Source1:      firefox.sh
 Source2:      firefox.pod
-Source3:      about.png
-Source4:      aboutCredits.png
+Source3:      firefox-init.tar.gz
+Source4:      about.png
+Source5:      aboutCredits.png
 Patch0:       firefox.patch
 
 #   build information
@@ -257,6 +258,12 @@ AutoReqProv:  no
     ( cd dist/bin && %{l_tar} -chf - . ) |\
         ( cd $RPM_BUILD_ROOT%{l_prefix}/lib/firefox && %{l_tar} -xf - ) || exit $?
 
+    #   add initialization files
+    ( cd $RPM_BUILD_ROOT%{l_prefix}/lib && \
+      %{l_tar} -xf %{SOURCE firefox-init.tar.gz} && \
+      rm -f firefox/README
+    ) || exit $?
+
     #   add startup convenience wrapper
     %{l_shtool} install -c -m 755 %{l_value -s -a} \
         %{SOURCE firefox.sh} $RPM_BUILD_ROOT%{l_prefix}/bin/firefox