Firefox uses a "benevolent dictator" model for decision making. This role is performed by the Module Owner.
Within the Firefox module, the Owner and Peers lead development of specific sections of code, reviewing patches and pushing things forward.
The current owner and peers for Firefox are listed on the Mozilla Module Owners page.
Within each project there are generally teams of folk that handle coordinating each release (both major and security update). These teams consist of Owners/Peers/ drivers/QA representatives etc. These groups may make decisions that impact the shipping product (which bugs are approved, etc).
Generally, consensus is reached around specific designs and development continues. Where consensus is not reached, it is the responsibility of the Owner to guide the discussion (and make decisions, if necessary) so that consensus can be reached and things can move forward.
If you have ideas, take them to the mailing lists/news groups. No owner or peer is going to approve your significant idea or feature patch for checkin before there has been discussion in an open forum. (Although obviously if you're just fixing individual bugs then this doesn't need to happen).