One of the main problems with upgrading versions of a browser – especially beta versions – are that existing plugins are not always supported. Frequently this means losing functionality while developers implement the changes necessary to get them working again.
Personally I don’t use many as I find it loads down the load times for the program; but the ones I do use I rely heavily upon.
Del.icio.us is one such example, and when the upgrade to Firefox 3 beta 4 resulted in the loss of 75% of my plugins it was …