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.

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.