ROOT does not start/work, or prints error messages about missing/wrong shared libraries during startup

Please remove or rename the files .rootrc and rootlogon.C in your home directory and the current working directory, and also make sure that the environment variables LD_LIBRARY_PATH and LD_PRELOAD are unset or at least definitely point to the right shared objects for the ROOT version you're using. If ROOT now works normally, you'll have to find out what in your startup files is causing this behaviour, or why your environment variables point to the wring shared libraries.

When linking against cernlib, I get lots of "missing _gfortran..." errors

Starting with SL5, the default Fortran compiler is gfortran and no longer g77, and the cernlib installed on SL5 was built with and for that compiler.

Standard disclaimer: cernlib is obsolete. We provide it in order to allow legacy projects to use a contemporary platform. It should not be used anywhere else, and it won't be around forever.

Firefox refuses to start, claiming it's already running, but it isn't

Run the command find ~/.mozilla -name '*lock' and delete the files found. Then try again.