There's a browser identification menu for spoofing for specific sites, but how do I change the user agent for all sites?
The default UA generation is hardcoded and you cannot adapt it without changing the source and recompiling.
However, depending on what you attempt to do the following work-around may be acceptable. The site/domain specific UA filter matches domain names part-wise. The consequence is that you can enter a top-level domain name and the specified UA works for all sites in that domain, e.g. enter