From time to time, I got this message ‘Bad Request: Your client has issued a malformed or illegal request’ when I was on Google’s web sites. The cause of this problem is because the browser’s “cookie” for Google has been expired or bad. The solution is to clear the bad cookie.
One Firefox, you have to click on Tools -> Clear Private Date and select Cookies to clear the cookies. This solution is not ideal, because it clears out ALL the cookies including the cookies for other sites that you might want to keep.
I found a plugin that does what I need, that is removing cookie(s) for a particular site. This is the page for the add-on. After installation, restart Firefox. Now, if you want to remove cookies for a site, just right click a space on the page and you can see an option to “Remove Cookie(s) for Site“.
There is also a toolbar button that you can add to your toolbar. Right click on a toolbar and select “Customize”. Drag and drop the cookie button to the toolbar. The button is helpful for those web pages which have right click disabled, or for those Flash based pages. Note that when you click on the button the cookies for the site got deleted immediately. So, be careful about it.