Auto merge of #18040 - atouchet:user-agent, r=jdm

Update Firefox version in user agent

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GitHub has started throwing warnings about using an outdated version of Firefox when loaded with Servo. I am assuming that this is due to Servo using Firefox 37.0 in its user agent which was set in 2015 and hasn't been updated since then. I don't know if updating this should cause any site compatibility issues or not.

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