Greetings to all !
Further to my last post I submitted this question to the linux Mint forum. I got a couple of answers back but neither directly solved the problem. However, they lead to further experiments and I have now found the cause.
I run my browser and email client (Thunderbird) in a sandbox, using the "firejail" utility. If the condition of default browser is set whilst the application is running sandboxed then, the setting will be lost on exit because firejail stops anything being written outside the sandbox, except to the directory "Downloads".
If Opera is run outside the sandbox then the condition "default browser" can be set and will persist after exit. However, Opera running in a sandbox is (functionally) a different entity so, one cannot set the default condition and hope it will be preserved inside the sandbox. The solution to this probably requires a different custom profile for Opera and I will probably develop this at some future date.
I hope this report will be useful in some way to others.
Best regards to all ---
I've read your are on Linux, because of that, this:
[...] At least this option exist and works in Windows. [...]
Now I know, it's a just Windows options.
If there were any difference between versions, an Standalone install could do the trick too.
[...] Vivaldi. This browser offers a lot of options for customising [...]
Yes, Vivaldi it's plenty of options.
It's the Chromium version of Opera Presto.
So, go for Vivaldi.
Maybe in the future, Opera becomes customizable.
Thank you for your reply.
I'm disappointed but --- "No big deal". I have had a work-around right since Day One --- one of my top level tiles in Speed Dial is reserved for search engines (Start Page, Duck Duck go, scientific databases, etc.). So anonymous searching is available via two mouse clicks instead of one, as would have been the case had it been possible to customise the Search Box.
Now that I am retired (I'm 83) I spend a lot of time on-line researching scientific topics so I like my browser to be customised to my personal idea of how the interface should look --- which is why I have stuck with Firefox for so long. With that said, Opera is very nice and I have presently got it set as default but Firefox is still available via a launcher on my Desktop. One of the things I admire about Opera is the way it imported my bookmarks from Firefox. I have well over a thousand of these, all in folders, several layers deep and Opera has preserved the organisational structure. This is a huge bonus point, so far as I am concerned.
Thank you once again --- Best regards from the UK