Setting the browser agent in Firefox allows the browser to pretend to be something else.
Traditionally the user agent string was changed to make the browser appear to be IE6. Without this some websites wouldn’t work.
FireFox hides the User Agent configuration in the about:config settings.
I guess is one of those less frequently configured options – why clutter and confuse the existing configuration options with an item such as this.
To set the user agent string in Firefox. Enter the configuration using about:config in the address bar.
There’s a confirmation that changing the settings can be harmful to the application and that you should only continue if you know what you are doing.
The configuration parameter to be modified is general.useragent.override. To see if its available enter useragent in the search box.
Its likely that its not listed as a configurable option.
To add it right click in the window (I placed the mouse within the white space below the listing).
On the pop-up menu select New > String
add general.useragent.override as the name of the preference.
The user agent can then be added.
You may prefer to use an Add-On to manage the User Agent. This will allow a list of agents to be maintained with easy switching. Much simpler to checkout websites than to keep returning to the about:config options.
With your FireFox browser configured to allow easy switching off the user agent you are able to test the reason of website to different browsers and bots.
Of particular interest maybe the response of one of your websites to different robots.
One of the website hacks (so called Pharma Hack) is to detect whether the visiting browser is the google-bot or a human visitor. The content of the website is then amended accordingly. If it’s the google-bot then it looks like it’s showing and referencing pharmaceutical products.
Maybe you have configured your website to block the bad robots by their user agent, for example one of the spam bots.
To test whether your website is rejecting these bad bots you can configure Firefox to appear to be one of them by setting the user agent string.
To review and check that your website is presenting itself as you would expect change the user agent in your Firefox browser and visit your website.