Contrary to the rumors, Mozilla Thunderbird usage is not declining. In fact, usage is still increasing steadily.
Mozilla measures program usage by Active Daily Installations (ADI), which is the number of pings that Mozilla servers receive as installations do their daily plugin block-list update. This is not the same as the number of active users, since some users don’t access their program each day, and some installations are behind firewalls. An estimate of active monthly users is typically done by multiplying the ADI by a factor of 3.
There is a more detailed breakdown of the growth, plus the top 10 countries in ADI (United States is 3rd) on the The Mozilla Thunderbird Blog.