It's just a suspicion, as stated. The reason for this though is that Firefox has (or had) the exact same behavior in older versions I've used prior to switching to Conkeror.
I was thinking this was going to be your argument, which is why I pressed the issue. If you have a reproducible testcase where Firefox without plugins ends up "hogging system resources", then by all means report it.
Do you have any actual data to support that claim (or suspicion, as you want)?