JavaScript is enabled by default - you can check that in 'about:config' , where the value of the preference 'javascript.enabled' should be set to 'true'.

But I think that you are talking about 'Java'  :

Java does no longer work on the release version of Firefox. The Extended Support Release (ESR) of Firefox 52 will still run Java and other old plugins. ESR was first developed to meet the needs of large organizations that wanted Firefox to change much more slowly. It gets security updates but no feature changes for about a year at a time.

More information on this change and ESR: Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work ?

https://support.mozilla.org/en-US/kb/npapi-plugins

Firefox Extended Support Release (which I gather from the header of your question you already installed) :

https://www.mozilla.org/en-US/firefox/organizations/ https://www.mozilla.org/en-US/firefox/organizations/notes/ https://www.mozilla.org/en-US/firefox/organizations/system-requirements/

Thanks - my query obviously reflected my ignorance. But it arose because the BBC Weather website wasn't showing maps etc and told me I needed to reinstall Javascript (but may have said Java - I don't recall). It directed me to a website that guided me through reinstalling it, though I then had to actiovate it also.

mikebrw said

Thanks - my query obviously reflected my ignorance.

Don't sell yourself short - it's very confusing Java / JavaScript  ! Very understandable to get those two mixed up  !