mjhjmtu

In the emacs' .emacs:

'(browse-url-browser-function (quote browse-url-default-browser))

I have restarted the system after setting Opera as default browser.
When I run:

update-alternatives --config gnome-www-browser

There are 4 choices for the alternative gnome-www-browser (providing /usr/bin/gnome-www-browser).

 Selection Path Priority Status
------------------------------------------------------------
 0 /usr/bin/google-chrome-stable 200 auto mode
 1 /usr/bin/firefox-esr 70 manual mode
 2 /usr/bin/google-chrome-stable 200 manual mode
 3 /usr/bin/iceweasel 70 manual mode
* 4 /usr/bin/opera 120 manual mode

Press <enter> to keep the current choice[*], or type selection number: 

and

update-alternatives --config x-www-browser

 There are 6 choices for the alternative x-www-browser (providing/usr/bin/x-www-browser).

 Selection Path Priority Status
------------------------------------------------------------
 0 /usr/bin/google-chrome-stable 200 auto mode

 1 /usr/bin/firefox-esr 70 manual mode
 2 /usr/bin/google-chrome-stable 200 manual mode
 3 /usr/bin/iceweasel 70 manual mode
 4 /usr/bin/konqueror 30 manual mode
* 5 /usr/bin/opera 120 manual mode
 6 /usr/bin/qupzilla 80 manual mode

Press <enter> to keep the current choice[*], or type selection number: 

When I click on a .html file directly, it's opened using Opera.

cat ~/.local/share/applications/defaults.list

[Default Applications]
application/pdf=FoxitReader.desktop
application/ppdf=FoxitReader.desktop

However, emacs insists on opening files with firefox. Where is it taking this setting from?

Please try:

(setq browse-url-browser-function 'browse-url-generic
 browse-url-generic-program "opera")