See Bug #112384 https://bugs.launchpad.net/ubuntu/+source/liferea/+bug/112384/comments/3

(bug disapeared after a rebuild without firefox-dev)

here is the relevant part of configure for that build:

checking pkg-config is at least version 0.9.0... yes
checking for LIBNOTIFY... yes
checking for LUA50... yes
checking for LUA51... no
checking for XULRUNNER... no
checking whether we are using the GNU C++ compiler... (cached) yes
checking whether g++ accepts -g... (cached) yes
checking dependency style of g++... (cached) gcc3
checking for GTKHTML2... yes
checking for MOZILLA... no
mozilla: no
checking for MOZILLA... no
firefox: no
checking for MOZILLA... no
seamonkey: no

====
config.log :

configure:24281: checking for MOZILLA
configure:24289: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package mozilla-gtkmozembed was not found in the pkg-config search path.
Perhaps you should add the directory containing `mozilla-gtkmozembed.pc'
to the PKG_CONFIG_PATH environment variable
No package 'mozilla-gtkmozembed' found
configure:24292: $? = 1
configure:24307: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package mozilla-gtkmozembed was not found in the pkg-config search path.
Perhaps you should add the directory containing `mozilla-gtkmozembed.pc'
to the PKG_CONFIG_PATH environment variable
No package 'mozilla-gtkmozembed' found
configure:24310: $? = 1
No package 'mozilla-gtkmozembed' found
configure:24338: result: no
configure:24350: result: mozilla: no
configure:24357: checking for MOZILLA
configure:24365: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package nspr was not found in the pkg-config search path.
Perhaps you should add the directory containing `nspr.pc'
to the PKG_CONFIG_PATH environment variable
Package 'nspr', required by 'XPCOM', not found
configure:24368: $? = 1
configure:24383: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package nspr was not found in the pkg-config search path.
Perhaps you should add the directory containing `nspr.pc'
to the PKG_CONFIG_PATH environment variable
Package 'nspr', required by 'XPCOM', not found
configure:24386: $? = 1
Package nspr was not found in the pkg-config search path.
Perhaps you should add the directory containing `nspr.pc'
to the PKG_CONFIG_PATH environment variable
Package 'nspr', required by 'XPCOM', not found
configure:24414: result: no
configure:24426: result: firefox: no
configure:24433: checking for MOZILLA
configure:24441: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package seamonkey-gtkmozembed was not found in the pkg-config search path.
Perhaps you should add the directory containing `seamonkey-gtkmozembed.pc'
to the PKG_CONFIG_PATH environment variable
No package 'seamonkey-gtkmozembed' found
configure:24444: $? = 1
configure:24459: $PKG_CONFIG --exists --print-errors "$gecko_provider-gtkmozembed"
Package seamonkey-gtkmozembed was not found in the pkg-config search path.
Perhaps you should add the directory containing `seamonkey-gtkmozembed.pc'
to the PKG_CONFIG_PATH environment variable
No package 'seamonkey-gtkmozembed' found
configure:24462: $? = 1
No package 'seamonkey-gtkmozembed' found
configure:24490: result: no
configure:24502: result: seamonkey: no

this is with:

ii  firefox                      2.0.0.3+3-0ubuntu2           lightweight web browser based on Mozilla                                   
ii  firefox-dev                  2.0.0.3+3-0ubuntu2           Development files for Mozilla Firefox                                      
ii  firefox-gnome-support        2.0.0.3+3-0ubuntu2           Support for Gnome in Mozilla Firefox                                       
ii  libnspr-dev                  1.firefox2.0.0.3+1-0ubuntu2  Netscape Portable Runtime library - development files                      
ii  libnspr4                     1.firefox2.0.0.3+1-0ubuntu2  Netscape Portable Runtime Library                                          
ii  libnss3                      1.firefox2.0.0.3+1-0ubuntu2  Network Security Service Libraries - runtime                               

I don't have nspr.pc but I do have firefox-nspr.pc (firefox-dev) and mozilla-nspr.pc (libnspr-dev).