How to change Firefox's user agent
You can change your browser's user agent so that your pc is being identified as another device like a mobile device. Identifying your browser as running on a mobile device will result in the mobile version of web pages being displayed (if it is available). This can be useful for numerous reasons: Mobile pages are smaller, loads quicker, you save a lot of data, a great way for older machines to access resource hungry websites, etc. Take youtube for example. The mobile site loads quickly, there are no waiting periods for ads to run, comments and other things can still be accessed, etc. So in this example we will change the user agent so that your device setup is identified as that of an iPhone. There are tools available to do this but this is the manual way:
For Firefox:
I didn't check newer Firefox but I changed the user agent of the Light Browser (which is an older version of Firefox) as follows:
type about:config in the addressbar > accept the risk to go past the warning messages > type useragent in the search area > check if there are any general.useragent.override entries. If not, create a new one by right-clicking and selecting New string and general.useragent.override as name of the entry and then setting its value to: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543 Safari/419.3
Close the browser and restart it. The changes can easily be undone by right-clicking on the entry you have made/changed and selecting Reset.
In the Palemoon browser I'm using, there are already general.useragent.override entries for a number of specific sites so you either change the value of some existing entries you want to change using the value as mentioned above or create a new entry as described.