[Note added 2009-07-19: The advice given in this post is now out of date. In modern versions of Firefox it is possible disable Quick Find using the menus. On windows, click: Tools > Options > Advanced > General and deselect Search for text when I start typing. On Linux click: Edit > Preferences > Advanced > General and deselect Search for text when I start typing.]
Recently I have found that when I start typing into a text box in Firefox I often get a Quick Find bar coming up at the bottom of the screen. This happens if I am typing ahead. Firefox seems to be interpreting characters typed at a web-page as a search string. I don't remember having asked for this behaviour and I find it rather annoying. I would much rather have Firefox silently ignore the type-ahead characters. At least then I would not have to go to the trouble of removing the unwanted Quick Find bar (or sitting and waiting until it removes itself). Fortunately a little research revealed the following fix:
Enter about:config in the Firefox URL line and click Go.
Select the line starting accessibilty.typeaheadfind and double click it until the Value field is false.