Problem building Firefox 35.0: build freezes

I have not seen this particular problem before. The build freezes immediately after leaving the directory '/usr/ports/opt/firefox/work/src/mozilla-release/firefox-shared/js/src/shell'. Normally leaving any build directory would be followed immediately by entry into the next one. When I checked the hard drive LED, it seemed to be jammed on, hardly flickering. In all my earlier FF builds, the only significant pause was during the linking of xulrunner. Hazel Russman