For the sake of completeness, I have included the complete fix to the current package version, resembling the successive application of the patches from Debian bugs #244305 and #330966.
diff -urN firefox-1.5.dfsg+1.5.0.1~/widget/src/gtk2/nsWindow.cpp firefox-1.5.dfsg+1.5.0.1/widget/src/gtk2/nsWindow.cpp --- firefox-1.5.dfsg+1.5.0.1~/widget/src/gtk2/nsWindow.cpp 2005-12-21 02:38:20.000000000 +0100 +++ firefox-1.5.dfsg+1.5.0.1/widget/src/gtk2/nsWindow.cpp 2006-02-09 11:08:03.000000000 +0100 @@ -1543,6 +1543,9 @@ return; switch (aEvent->button) { + case 1: + eventType = NS_MOUSE_LEFT_BUTTON_DOWN; + break; case 2: eventType = NS_MOUSE_MIDDLE_BUTTON_DOWN; break; @@ -1550,7 +1553,7 @@ eventType = NS_MOUSE_RIGHT_BUTTON_DOWN; break; default: - eventType = NS_MOUSE_LEFT_BUTTON_DOWN; + return; break; }