If I start the addons dialog and click some of the extensions (or whatever tab is open), the selection background is blue with a gradient. If I change to another component (themes for example) and click the available items, the selection color is darker blue (= the GTK selection color without any gradient). This selection color stays until I press the active icon *again* (e.g. select theme1 and press theme1 again): the selection turns back into the lighter blue with gradient.
I can confirm this as well too (Ubuntu 8.04 / Firefox 3 RC2) It's also strange because for me, when after I switch from Themes to Addons (for example), it might take a few clicks for the regular gradient to come back. It also occurs when you press the find updates button.
I can no longer reproduce this (we also no longer a gradient in this context), so marking as WFM.