This bug is with the 1.0.6-1ubuntu1~5.04ubp version of firefox.
When the auto-fill feature is enabled (that is, when Firefox remembers things you put in forms), this is reproducible if you can find a form that offers you both a select box and a text input field. I am able to reproduce this when I attempt to create a new table with phpMyAdmin's newest version.
Let us say, for example's sake, that the text input field has a value of "60" stored. If you type 6 in this box, then attempt to click the dropdown select box (not another text input field), the dropdown below the text input of 6 will disappear only until you release the mouseclick. Once you have released the click, the dropdown will reappear. In this way, you cannot select certain other elements on the page by the mouse if you are filling in a form and the auto-fill memory is suggesting a value to you (in the above case, the auto-fill memory would be suggesting 60 to the 6 you typed).
I have not found any code issues that cause this problem, though it is easily reproducible with the following code for a form:
<html>
<head>
<title>Form Bug Test</title>
</head>
<body>
<form method="post" action="">
<input type="text" size="30" name="field" />
<select name="dropdown">
<option value="1">This is an option</option>
<option value="2">This is another option</option>
</select>
</form>
</body>
</html>
With that code, fill the text box with a bunch of random characters such as "aaaaaaaaaaaaaaaaaaaaaaaaaa", then hit enter. Press Back, then clear the text box, type one "a", and you'll see the dropdown appear. This behaviour is enabled in Firefox by default. Now try to click on the dropdown and you will experience the bug.
not a MOTU bug, since it is a backport package.