CFW doesn't really mean "all new firmware", just that it's custom and different from stock. Most CFWs are just the stock firmware with custom patches applied and packaged neatly for end-users to install.
You can check with this list for a chart of PS3 models:
https://www.psdevwiki.com/ps3/SKU_Models#Retail_Models
I recommend getting a PS3 slim 21XXX and 25XXXX because they use the smaller RSX (failed very often in older models - specifically the fat models). Not all 25XXX models are downgradable to 3.55, just early ones. I'm not sure on which ones, but I've heard early to mid 2010 models are 3.55 compatible and anything late in 2010 or 2011 are HEN only.
https://www.psdevwiki.com/ps3/Datecode