web browser goes slow when opening a web site with a flash file

Which flash do you have? the flashnonplugin-free package or gnash or swfdec.

Look into firefox and put 'about:plugins' without the quotes in the URL .

The first thing you will see are of flash.

This is mine.

Shockwave Flash

    File name: libswfdecmozilla.so
    Shockwave Flash 9.0 r999

MIME Type Description Suffixes Enabled
application/x-shockwave-flash Adobe Flash movie swf Yes
application/futuresplash FutureSplash movie spl Yes

As one can see from the filename its swfdec-mozilla which I'm using.

Shockwave Flash

    File name: libflashplayer.so
    Shockwave Flash 10.0 r15

MIME Type Description Suffixes Enabled
application/x-shockwave-flash Shockwave Flash swf Yes
application/futuresplash FutureSplash Player spl Yes

there u go, and i want to add that my browser goes slow when i am in the tab that contains a flash file, when i have multiple tabs opened and i go to other tab it goes to normal speed, but when i return to the tab that contains a flash file, oh my god it goes too slow.

Many bad Firefox behaviors are in fact caused by one of the many third-party 'themes', 'extensions' and 'plugins'. These will go away with a new profile - that is, when Firefox is restored to default settings and without any extensions and themes. You do not need to delete your old profile to test.
Can you please try with a new profile:
https://wiki.ubuntu.com/MozillaTeam/Bugs?action=show&redirect=DebuggingFirefox#Try%20with%20a%20new%20profile

Also, there is a new version of Adobe Flash 10.0.22.87

ok, I'll try it!, and send you my feedback!

We are closing this bug report because it lacks the information we need to investigate the problem, as described in the previous comments. Please reopen it if you can give us the missing information, and don't hesitate to submit bug reports in the future. To reopen the bug report you can click on the current status, under the Status column, and change the Status back to "New". Thanks again!