"Is it true that at the majority of companies it is not performed"
Well I haven't done a survey, have you? As far as I know web development companies usually do Cross-browser Testing (C.B.T.). They have to make sure the websites and web applications they develop look and work in the same way in various browsers. This is to make sure that all users have a good experience on the site or application and want to visit again. Plus, they have to do this is because they don't control which browser and which version of that browser their end users will be using. The most they may do if an end user reports an issue is ask them to upgrade to the latest version. Mostly the companies test of few widely used browsers like FireFox, Chrome, Safari, Opera and I.E.
"since most of the time when something working on one browser and not
working on another, it is because something is not configured or
updated properly in one browser"
Unless an until your end user has some technical knowledge and requires specific configurations and plugins set up in their browsers, they will install the browser with default settings and use it that way only. So, no its not about "something is not configured or updated properly in one browser". Its about something wrong in you code.
"Is it true that even when it is worth doing c.b.t. any time spent on
it is time lost on functional testing and most of the defects have to
do with the code and little to do with a browser?"
No. Time spent on C.B.T. is not time lost on functional testing. If it is then the problem is with your analysis and estimation. You should have estimated C.B.T. and functional testing separately. "most of the defects have to do with the code and little to do with a browser" When you say C.B.T. are you testing your website or web application's compatibility with various browsers or testing the browser? Yes most defects have to do with problem with the code and very few occur due to problem with the browser!
"Would it be save to say that in automation testing c.b.t. is as
common (or rather as uncommon) as in manual testing?"
This totally depends on the individual thinking. If you are comfortable with writing automation script to test a website or web application of various browsers then it will be common for you.
"Is it true that if an application is tested in only one browser in
Selenium, it is standard to test it in Firefox?"
Selenium provide built-in support for FireFox. That is why people prefer FireFox. You can install and configure drivers for other browsers separately and then use them. But, with C.B.T. people usually test the design compatibility more than the functionality. Selenium will help you test the functionality of your website or web application and not the design.