The fire flower on a topic.

[RudeGuy]

If there is a flower near the name of the topic it means the topic is sticky.

[SuperWingedYoshi]

Huh? No, wait! A fire flower appears if there are 25 or more replies in that topic. It marks popular topics.
Stickys, which are important topics that are always at the top of the page, have a green or a red Koopa shell.

[Raster]

If there is a flower near the name of the topic it means the topic is sticky.

That is wrong. Fire Flowers means it's a Popular Topic. Like smwforever45 said, the topic must have over 25 replies.