No idea what's wrong, the download button appears for me.

You can download all localized versions from here.
http://www.mozilla.org/en-US/firefox/all.html

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot

Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (1 MB).

Make sure that you allow pages to choose their colors and that you haven't enabled High Contrast in the Windows Accessibility settings.

• http://kb.mozillazine.org/Website_colors_are_wrong
• http://kb.mozillazine.org/Websites_look_wrong

Ok, now I have downloadet firefox, but I cant use it because my OS X is older than 10.5 (I use 10.4.11). I cant find a free version of 10.5.0 anywhere - any suggestions?

As you noticed Firefox 4 and later require at least OS X 10.5 and an Intel Mac.

You are unfortunately out of luck with an Intel Mac and OS X 10.4.11 and best would be to update to OS X 10.5 or later if you want to use current Firefox 5 and later versions.
There is a Firefox 4 compatible third-party version TenFourFox from www.floodgap.com for a PowerPC Mac, but no alternative for an Intel Mac with Mac OS X 10.4.

Firefox 3.6.x (Mac OS X 10.4 and later) can be found here:

• http://www.mozilla.com/en-US/firefox/all-older.html