When i add plain text inside pre tag it's formatted( space changed to line-break to <br /> and so on). When i add it in html view - all spaces and line-breaks are deleting too. It seems to me unnecessary.
How can i stop TinyMCE doing this, save original formatting inside <pre> block?
It is formatted depending on your browser. You won't be able to change that- and if you are you shouldn't do it.
In Firefox the br-element is necessary to be able to click inside the html element, so is the .
Spaces added in html view at the end or beginning of a html-element get removed (trimmed out) by default - that is correct btw.