I made this a while ago but forgot to post it here, then the redesign happened and it broke, and now that I've fixed it I'm finally sharing it with you guys.
Hotkey is set to Shift + F. The script automatically both marks as favorite a post and upvotes it.

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Downloads

GreasyForky
Direct Link
Direct Link [Mirror]

You need Tampermonkey for this.

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Thank you so much!

You can press "F" to do the same thing without a script assuming you have keyboard shortcuts on in the settings. Plus other useful stuff.