Are you looking for : about:config > I'llbe careful,I promise > enter Timeout in search bar and hit enter. You will see a number of preference list with there value . The top two are disable timeout and the "count timeout".

If you want to disable timeout, simply double click the "enabletimeout" on the top. This will change the value to false. If you would like to change the value of how long it takes to timeout, double click the "CountTimeout" and enter in your new value.

• excerpt from https://support.mozilla.org/en-US/questions/1042479

If you mean the screensaver from the Windows operating system then you need to check its settings via the Windows Control Panel.

Otherwise you would have to rephrase and give more details about what you mean.