check in its settings you will find it

which settings do you mean? Tools /Options / language is set to English - it would be great to know if there is something else to change to sort this language display problem out

The preference name is general.useragent.locale. Note that this will only affect add-ons; the Firefox interface will remain in the same language, unless you install an appropriate language pack.

By default, add-ons will display in the same language as your Firefox language, if possible. Some add-ons are not translated into certain languages, and will therefore only display in English.

If you want Firefox and add-ons in English, the simplest thing to do is to download it from the following page and install it on top of your current version.

• www.mozilla.org/firefox/all.html

If you want to switch between multiple languages, you can install a language pack

• Language packs for Firefox 11 on Windows

Once a language pack is installed, you can switch between languages either using an extension like Quick Locale Switcher or by manually configuring the general.useragent.locale preference.

On an unrelated note, your version of Flash Player is outdated. It would be best to update to the latest version, to avoid any potential security risks.

• get.adobe.com/flashplayer/otherversions/

thanks Gingerbread man, appreciate reply - I have the English version of firefox, so no problem with that - it's just that some addons are not in English. I have already changed general.useragent.locale - language is set to en-GB, so I expected addons to use that value to set language.

Apparently not though...since so many addons seem to use some other value to pick up my location in netherlands and set language to Dutch. I have contacted some of the plugin developers to see if I can find out anymore about what is going on and see if there is an option to change.

There aren't that many extensions (if any?) that directly support the en-GB local because en-US is regarded as the default English version.
Extensions usually fall back to en-US if there is no suitable locale, but Firefox inquires the OS for metrics including the default language and that can cause the Dutch nl locate to get get selected.
So the en-GB locale setting can be a possible cause for getting the Dutch version in extensions that do not support "en-GB", but do support the "nl" locale.
You can try if it works with setting the general.useragent.locale pref to en-US

thanks - good suggestion, but unfortunately also didn't work! changed general.useragent.locale pref to en-US and reinstalled some addons (greasemonkey, fireshot) both still in Dutch. I'll have to try on some other computers / OS and see if there is the same problem - won't have time to do that until later in the week - will update on results when I have had the chance to look into this some more

I never found out what was causing the addon language problems, but I finally solved this in the end by re-installing the US version of firefox.

Thanks everyone for all the replies to try and help get this fixed!