I have this problem with Google Search in Firefox (in both Linux and Windows version). After some (mostly one) search queries with google.com, if I decide to change the keywords (in the input search of the page, not the address bar of Firefox), sometimes google sends me the file named f.txt for download, under the search box is this line "Google Instant is unavailable. Press Enter to search". Then I have to copy the search query to the address bar of Firefox to continue to search. It seems like this problem is caused by Firefox only. I tested with Chrome, IE and Google worked correctly.