I swear, if developers keep adding decimals to their versioning systems, I might just have to start a petition or something. Firefox 18.104.22.168.22.214.171.124 is available with "improvements to product stability" and several security enhancements. I believe it should be available via Firefox 1.5's built-in updating system, or simply from Mozilla's Firefox page.
[note: extra '.126.96.36.199' added for humor. Not intended as a real software versioning system - yet]