The browseTimer is a countdown clock that appears in the bottom-right corner of the Firefox status bar that lets you self-impose a limit on your web-surfing time. Set the time and hit the Start button (5 minutes is the default start time).
Here is a screenshot:
When the timer reaches zero, all of your Firefox tabs (in all Firefox windows) will be emptied of content and the background color will turn red. No tabs will be closed, so if you meant to bookmark a tab before time ran out, it'll still be open for you to do so.
There's nothing to stop you from reloading the affected tabs and restarting the timer: it's up to you as to whether or not you want to abide by the time limit you set for yourself.
Due to slight differences in how the input boxes are rendered in the Macintosh and Windows versions of Firefox, there is a Macintosh and a Windows version of the add-on. Click the appropriate link to install the add-on to your browser (you will get a prompt at the top of the page):
If you decide you don't like the add-on, you can install it by opening the Jetpack console (type "about:jetpack" in the navigation bar), clicking on "Installed Features", and clicking on the "uninstall" link next to the entry for browseTimer.