This problem occurs when you have finished the update and try to browse using firefox. The problem occurs in the absence of a font that is supposed to depend on our ubuntusystem. To fix it, we can use the font setting preferences so that there is no clash.

Just follow the following commands.

Click edit>preferences

on tab "default font", change to "ubuntu" , and then click "advance".

 

Uncheck "Allow web to change that font".

Ok, finish. Now your mozila firefox browser is come back.