Browser extensions
FoxyProxy
In some assessments/pentests, it is important to check applications from different geolocalisations, and it is possible to configure a public or private VPN in FoxyProxy.
User-Agent Switcher
Some applications work differently depending on web browser and devices from which they are opened. It is not possible to have all browsers, versions, and devices to test them. But, all of them are identified by a parameter included in the HTTP request, the User-Agent.
It can be switched directly in the HTTP request using an HTTP proxy, but to make lives easier, use the User-Agent Switcher, a Firefox add-on that has a lot of user agents to analyse different responses.
HackBar
HackBar is an add-on that integrates a simple HTTP proxy into Firefox, providing the capability to manipulate HTTP requests. Only use for simple testing.