Ever since I use waterfox, I get that 403 on the entire forum. I think it's some sort of a spam protection thing. The solution is to change your user agent string to something that is whitelisted, such as...
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:25.0) Gecko/20100101 Firefox/25.0
In the case of firefox you can change that somewhere in about:config.
(dunno if this post will be useful since it only happens occasionally for you... but these are my 2 millibitcoins anyway)