Clicking on URLs often ends up downloading the webpages to my disk instead of bringing me to the page. I have 33 copies of Coffeehouse.htm in my Downloads folder, and only as little as that because I've deleted some already.
I hate this too, it occurs when you click on the textfield and then move the mouse downwards, thus making the previous autocomplete entry selected. When you press return it submits the selected entry. I try to move the mouse up, or not at all, after clicking.
Even though there's a dialog for automatic installing for plug-ins, it always makes me do a manual install.
What exactly do you mean? I can go to Tools > Options > Add-ons > Select an add-on and then add it to Firefox unhindered. No problems when using the add-ons website either.