I have already had both of the mentioned extensions for a long time now. I happily endorse their usefulness... I'd considering switching to Firefox just to have these if you run IE.
An addition to the timer in the window: If you double-click it, you get more. It brings up a window showing guild opening/closing times, airship arrival/departure times, upcoming Ballista matches, RSE timers, and even times/limits/rewards for the current chocobo delivery quests; all of these are in realtime (running counters) and are displayed in Earth time.
Neither creates lag, so don't worry about your browsing speed slowing down.
These are freakin' awesome. =D
Edited, Thu Feb 2 18:14:50 2006 by Speedly