I saw many people using TamperMonkey extension especially for cool Firefox css features but does it affect my privacy and security? If yes, How to use it without its cons?
Technically, no until you will add some malicious script.
But you should also keep in mind that all extensions can be used to fingerprint your browser
PG only recommends uBlock Origin as other extensions will make your browser more fingerprintable and add to attack surface.
It comes with neither upsides, nor downsides on its own (besides the obvious part where it’s another vendor you’re trusting with not straight up serving you malware).
The others here are mentioning fingerprinting, as far as I’m aware this is untrue. You cannot detect Tampermonkey, only possibly scripts (if any) that are injected into the webpage. That is, unless you use a userscript that covers that domain, it makes zero difference.
As for security and privacy of userscripts - that’s something you need to decide individually per script.
Yes, if it’s injecting other CSS or modifying the page it will affect the fingerprint. Anything which modifies requested resources or changes page layout will.
This extension requires a lot of privileges to run as well.
I wouldn’t use TamperMonkey in the frist place. It’s closed source. Why not use an open source alternative like Grease Monkey instead?
and tbh, if you have to use something like this for debugging Violentmonkey is probably a better choice.