Enter about:config in the url location bar and hit enter.
Right-click anywhere within the list of preferences and select New -> Integer.

When you get asked to enter the preference name, enter:
browser.bookmarks.livemark_refresh_seconds

You will then be asked to enter a value.  Set the value to whatever refresh rate you’d like. The unit for the value is seconds, so if you wanted the refresh rate to be 5 minutes, the value would be 300 (as in 300 seconds). The minimum is 60 seconds, so if you enter anything less than 60, it will still default to 60 internally.

Be careful when you set the rate too low, sites such as slashdot will refuse your request if you set the rate to 60 seconds. To be safe, I think 300 is a good value.