As you may know, Mozilla Firefox is Debian’s default browser, but the Debian developers have named it Iceweasel and kept this name for the last 10 years, due to a trademark disagreement with Mozilla.
Because there are many developers that work for both Debian and Mozilla, the conflict wasn’t real, but despite this, Firefox remained Iceweasel for 10 years. Along with Firefox, Thunderbird was also renamed to Icedove, from the same motives.
“Mozilla recognizes that patches applied to Iceweasel/Firefox don’t impact the quality of the product. Patches which should be reported upstream to improve the product always have been forward upstream by the Debian packagers. Mozilla agrees about specific patches to facilitate the support of Iceweasel on architecture supported by Debian or Debian-specific patches,”