Well, I've changed the default browser to
opera in the Preferred Applications:
firefox is still in the dash.
Chromium was the default browser, and it was in the dash after making it the default browser.
For example: I've changed The default music player to
Clementine and it didn't take time to show in the dash :
But the problem is still that Opera isn't showing in the dash, I think that there is something preventing
opera from showing in the dash.