Unless I missed it, nobody mentioned AppImages. A lot of projects are starting to release bleeding edge versions as AppImages and I've had good luck with that platform.

Most major projects provide either a .deb file that can be installed with gdebi or from cli or some like Firefox have an archive that can be extracted and used as a portable install.

All in all you need to research the application you are trying to install and see if they have a wiki or some other installation instructions. If that fails, open a thread on their forum(or LQ software subforum), or contact them via email.

If the program is NOT actively being maintained it a different story altogether.

BTW:
Go directly to https://ftp.mozilla.org/pub/
and click on thunderbird.

Of course, one will then have to know how to untar a bz2 file or use xarchive or such to install (which raises the question, where?)
Decisions, decisions. . . !

Actually this sounds like someone who, right at the beginning of their Linux journey, missed to familiarize themselves with some specialties of GNU/Linux, namely what Repository & Package Management means, and simply hung onto the only familiar paradigm they've ever known, which is: hunt, point and click.

When it takes all but concentrated reading of one web page and some additional minutes spent familiarizing oneself with some new software, usually synaptic, and you're done for a lifetime of enjoying Linux.

Application containers only play into this "hanging on to familiar paradigms" by adding a whole new layer of abstraction. They were not made for this purpose, but they enable droves of Linux newbs to hold on to their hunt, point and click mentality, and droves of equally enthusiastic podcasters and bloggers wax with admiration: "Finally, Linux will truly be like all the other $PROPRIETARY_OS out there! No more complaints about unintuitive UX etc.!"
...[insert long row of expletives here]...

Re #5.
The commands are :

Well said :)

One of the reason you can't just click an executable in linux/unix is security. If you need to understand what you're installing it's much more difficult to get viruses by clicking links, or hijacked executables.

that at the time, nobody had ever thought of that possibility. In the meantime, nobody felt the need to change that.

I'd say, one reason for the earth to still be round is that nothing has ever imposed something else. Not even security-considerations...

Of course there is no technical barrier to using and learning about Linux anymore. There used to be, back when you had to compile your own drivers to so much as get the display running.

IMHO I believe it's more a question of seeking help within the user community for specific issues - we come off as expecting users to have enough familiarity with what they're doing so as to be on that level. Specific distros of Linux *cough*Arch*cough* are some of the main offenders.

Even the general way in which one requests help with a topic is more suited to the mindset of a software programmer: have full log files ready, RTFM, STFW, methodically cross off every other possibility and then whatever you're left with - only if you have the experience to suggest it might not be the fault of your own settings or mis/configurations - might just be a problem worth looking into.

It's actually a wee bugbear of mine that we can sometimes scare off newbies on here. This happened in a thread in which I was participating just recently, where suddenly the OP was flooded with alternatives and information that went beyond solving the problem, and I'm sure merely led to confusion in their mind as to what to do next. We have to keep on remembering that newbies don't yet have tremendous knowledge and need "guiding" rather than being presented with lots of options and told "here, take your choice". It's almost akin to communicating with someone in a language that is similar but not quite the same as one's own - an effort has to be made to talk in that other person's language.

If we condone the deviation that this thread has been taking.
The expectations of new users nowadays are different. If we must adapt our way of reacting to this kind of critique, it must be in the awareness that the access of this kind of operating-system is demanded by people who are currently not apt to it. Else, define Linux.

Those responses in the thread, which I consider appropriate, direct the OP in the direction of familiarization with what he declares to be interested in. Else he/she would not, else she/he should not use Linux.
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