how does faculty search committee judge a research scientist applying for a tenure-track position Postdocs are considered on regular path for tenure track position. What about a candidate that did one postdoc and then moved to an associate research scientist position instead of a second postdoc? Are there any inherent bias against these candidates who applying for regular tenure-track positions? Do the faculty search committee evaluate them fairly ?
I don't see any reason based on what you have said to make any distinction between the two candidates. Not all post docs are a golden ticket. 

How much research counts at all depends on the institution, with it being only somewhat important at prestigious teaching colleges and the end all at top research institutions. I'll assume you are seeking a job at a research sort of place, R1 or R2 in the US. 

Depending on the field and specialty, people may have to depend on just counting things like papers and citations and looking at the reputation of journals and such. That might be the case if you were bringing something truly new to a department. But in other places, one or two of the faculty might actually read one or two of your papers, especially if you will be joining a specialized research seminar and especially so if you are joining a specialized lab with a clear focus. They will need to work with you and so need some sense about what you can do specifically. 

Do they judge them fairly? That is a harder question. Apples often have to be compared to oranges (kumquats) and a judgement call is often made. Sometimes, I've seen the senior faculty do a bit of horse trading. I'll vote for Sue Supersmart this round, but want your support for someone for my group in the next. It can actually be much more cooperative than that sounds, actually.