For Developers using Console2 and who like the convenience of using the command line for testing. I test on Nightly, and discovered that as of FF 28, using the -jsconsole flag on the command line no longer opens Error Console, and thus, no longer will open Console2 for those who use it. Instead, it opens FF's Browser Console, which may be an adequate substitute for some, but for those of us who still want to use Console2, I decided to create an addon which will allow the use of a flag (-jsconsole2) on the command line to open Console2.
It is true that using -console2 flag will open Console2, however, for some reason it blocks the opening of the browser window upon startup.
Start -> Run "%ProgramFiles%\Mozilla Firefox\firefox.exe" -no-remote -jsconsole2 -P dev
Start -> Run "%ProgramFiles(x86)%\Mozilla Firefox\firefox.exe" -no-remote -jsconsole2 -P dev
/Applications/Firefox.app/Contents/MacOS/firefox-bin -no-remote -jsconsole2 -P dev
/usr/bin/firefox -no-remote -jsconsole2 -P dev
/usr/local/bin/firefox -no-remote -jsconsole2 -P dev
console2 Command Line Argument is a very simple and lightweight addon (12kB on disk)