The Mozilla organization has released new versions of its Firefox browser and Thunderbird E-mail client; in both cases the new version number is 10.0.2. The updates fix a critical security vulnerability in the library used to render PNG graphics. More information is available in the Release Notes for Thunderbird and for Firefox.
I recommend upgrading your system as soon as you conveniently can. You can get the new version via the built-in update mechanism (Help / About/ Check for Updates), or you can download a complete installation package for Firefox or Thunderbird, ina variety of (human) languages.