Earlier this week, there had been reports of ads appearing on the twitter.com sidebar, but these new Firefox ad/messages spotted by Friendfeed user Alphaxion are without question the most prominent ad placements yet.

3703744315 e945066b72 New more prominent Twitter Ads Spotted

Techcrunch has also reported that some users had spotted the Firefox message above their streams. According to Twitter co-founder Biz Stone however:

“This is not an advertisement and it’s not just for one browser—it’s customized based on which browser you’re currently using.”

The ads/reminders are however appearing on multiple browsers from IE through to Safari, and not only that – but are reappearing a few refreshes later. It might very well be that the message is merely that, a reminder, and as Chris Sacca has reportedly indicated, its merely a matter of Twitter not being able to detect what browser the visitor is actually using.

We’re more likely to believe Marketing Pilgrim’s Andy Beal:

give it 6 months before ads start showing up in the exact same place–maybe even linked to the #hashtags you’ve clicked on. Oh wait, you didn’t realize that Twitter made #hashtags clickable, so it could track your interests?”

These are most likely a way to test engagement levels before deciding whether or not to fully integrate the ads. What I don’t understand is why, just as they were earlier with the sidebar ads, Twitter (and many other web companies) are so coy about potential new ad placements. The company has to start earning at some point and whether its through ads or confirmed reports of analytics tools being made available to business users…lets just hope that Twitter does what it needs to do to stick around without destroying it core selling point, its simplicity.