See http://support.mozilla.com/en-US/questions/795786

Maybe this add-on will help too: https://addons.mozilla.org/en-US/firefox/addon/default-fullzoom-level/

Thx for reply, but I didnt find any solutions in those links :(

When you say "zoom", do you mean full screen perhaps?

If so, hit the F11 key.

No, I mean that, when I right-click on some link and choose "open in new tab", that page where I clicked on the link will zoom-in a little (equals one-time CTRL+). That means, it keeps zoom-in when i opening link in new tab which ends up in extreemly big letters.

Do you have McAfee Site Advisor installed? If so, see the post at the bottom of this thread: http://support.mozilla.com/en-US/questions/785541

By the way, install this add-on to open links in a new tab without right clicking: https://addons.mozilla.org/en-US/firefox/addon/open-link-in-new-tab/

Also, try running Firefox in Use Troubleshoot Mode in Firefox.

Aaaah, finally solved. Yes I have McAfee site advisor. I figured out that I dont have to disable site advisor as stated in the link you posted, but only enable Add-on Bar (View-Toolbars-Show Add-on Bar). After that, no zooming occured. On the Add-on Bar are icons of McAfee though, so maybe McAfee was causing the problems somehow. It doesnt matter because it works now. Thank you Xircal, without your help, I would definatelly take some other web browser :)

You're welcome.