Just follow this simple steps:
1. go here http://www.facebook.com/friends/edit/?sk=requests
2. copy and paste either of this codes to your browser
a. for accepting all requests
javascript:for( i = 1;i<document.getElementsByName("actions[accept]").length;i++){document.getElementsByName("actions[accept]")[i].click();}void(0);
b. for rejecting all requests
javascript:for( i = 1;i<document.getElementsByName("actions[hide]").length;i++){document.getElementsByName("actions[hide]")[i].click();}void(0);
3. wait for all requests tobe accepted/rejected
DONE!
NOTE: For GOOGLE CHROME users, type javascript: first on your browser before you paste the script. or else it will only be a search query. :)