There is a way to get a custom firmware for this model that improve the video recording? Especially the autofocus, which is extremely slow.
Maybe there are some other good options in a custom firmware that I don't know of.
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I'm not aware of any software to improve the video autofocus performance of the Canon EOS 500D. In general, video recording was added to DSLRs an afterthought (except the Sony Alpha A33, A35), thus the usability generally suffers in comparison to true camcorders / video cameras.
So in light of that, you will have to use the method that all filmmakers live by: focus pulling - in other words, manual focus by hand.
They never made an alternative firmware for the 500D in the end. There is nothing that will improve the video recording for this camera.
You could however use something like MagicLantern for other tweaks to the cameras operation. Other cameras soon came out after the 500D with the improvements people wanted for it, so it seems people gave up on finding custom fixes for it.