The Alexa Fluor family of fluorescent dyes is produced by Molecular Probes, a subsidiary of Invitrogen. Alexa Fluor dyes are frequently used as cell and tissue labels in fluorescence microscopy and cell biology.
The excitation and emission spectra of the Alexa Fluor series cover the visible spectrum and extend into the infrared. The individual members of the family are numbered according roughly to their excitation maxima (in nm).
Alexa Fluor dyes are synthesized through sulfonation of coumarin, rhodamine, xanthene (such as fluorescein), and cyanine dyes. Sulfonation makes Alexa Fluor dyes negatively charged and hydrophilic. Alexa Fluor dyes are generally more stable, brighter, and less pH-sensitive than common dyes (e.g. fluorescein, rhodamine) of comparable excitation and emission, and to some extent the newer cyanine series. However, they are also more expensive. They are patented by Invitrogen (which acquired the company that developed the Alexa dyes, Molecular Probes).
The confusing part is how they are deciding the country to show. In my exploration i find out its showing the country where that specific site have better rank. Like for Asif2BD.info, it has Alexa rank of 750 in Bangladesh so its showing Bangladesh. But when i was exploring more i find out, its showing USA for facebook, where FaceBook.com have Rank 3 in USA, but Facebook has rank 1 in Bangladesh, but its still showing USA as showing country, so it seems its consider origin country too, like where its owner information or where is its server.
All Webmaster must have it, even normal people could have it, it shows WebSite ranking and page rank, it could help anybody to understand about visiting site. And it's not like annoying toolbar, it just show rank at the very bottom of the browser, it does not make browser slow. Its a must-have tool for everyone. Just try it on. BTW its great for SEO purpose.
This post is obsolete now, as Firefox discontinued Addons from version 57!
Here is details and direct download link from Alexa. And yes you must have Firefox. But I think all webmaster already got Firefox, the only webmaster with mac use Safari still. They should try Firefox with Alexa Tool bar, it's really worthy.