Coincidentally i'm writing this from my late 2009 iMac. It is already more than a decade old but i think it is a perfectly fine computer. With the latest version of Firefox every site works.
The main issue it has is that it is a bit sluggish but i think an additional 8GB of RAM (it has 4GB) and perhaps an SSD would make it feel perfectly fine.
Sadly Apple doesn't seem to agree and the last version of macOS to support it is 10.13 - which itself isn't supported anymore as of December 2020 (just ~3 years after it was released, which is kinda mad IMO). Most things seem to work fine so far (most open source applications seem to support even older versions anyway), though Homebrew (which i used to install a couple of command line tools) does warn that they have no official support for it and some stuff may break (fortunately that didn't happen).