Mozilla has released the next updates for Mozilla Thunderbird with versions 17.0.5 and 17.0.5esr (Thunderbird for Organizations) on April 2nd, 2013. Fixes in this version include:

• Adjusting font size when composing emails should be easier (Bug 824926)
• Various security fixes. 

Depending on the operating system and update settings (Tools > Options… > Advanced > Update Tab), users will be prompted to update within in the next 48-hours. Windows users can also go to Help > About Thunderbird and click the Check for Updates button. Users can choose to manually download and install Thunderbird via getthunderbird.com. The next scheduled release will be Thunderbird 17.0.6 and 17.0.6esr on May 14th, 2013.