This seems to be a bug in FCKEditor. There are several mentions about this in the editor forums, e.g. here
In KenticoCMS 4.1 version, we use FCK 2.6.3. The newest FCK (now CK) Editor version on the web is 3.0 and in the demo, the Blockquote works fine in FireFox. However it's a difficult process to integrate new version into KenticoCMS because we use our own dlls and functions.
We tried to find any workaround or fix to this, however without success yet, the editor code for Firefox seems to miss some definition(s).
Anyway you could enter the block quotes tag manually in the 'Source' view mode of the FCKEditor. When you hit the 'Source' button from the toolbar you switch the content to HTML source view. You could simply wrap required text with the '<blockquote>...</blockquote>' HTML tags so achieve what you actually mean.
I'd also suggest to contact FCKeditor community (currently it has been renamed to CK Editor), e.g. in the above linked forum...