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I love using the 'User Agent Switcher' Firefox addon on most sites on my desktop PC, in order to trick websites into using the mobile versions by masquerading as an iPhone/Android user agent. I like how on mobile sites I can enlarge text as much as I want and it will move text to the next line instead of making me scroll left and right to read it all.
However, of course, video sites such as YouTube ask me to install the Safari plugins, which I cannot do. I would love to be able to use the 'User Agent Switcher' addon in separate tabs, or only for certain websites.
How can I do this?