See how long your machine has been running

uptime

Example Output
23:42:28 up 6:20, 2 users, load average: 0.16, 0.22, 0.17

23:42:28 - Current Time
6:20 - The system was on for 6 hours and 20 mins

Installing the dependencies for HTML Validator extension for Firefox on linux.

sudo yum install compat-libstdc++-296 compat-libstdc++-33 tidy