You did not say what you wanted to update.

Download Firefox Full Version For All languages And Systems v39 {web link}

Find plugins here; https://support.mozilla.org/en-US/forums/support-forum-contributors/711401

I'm not sure the updater can handle that big a change - 29 versions. At any rate, downloading the full installation package is a smaller download than an "update" of more than one version would be. The updater handles one version change at a time; you skip one version and it "shifts" to a full uncompressed download - roughly 20% larger than the full installer which has compression to save bandwidth.

Full off-line installers are available here for the most recent Firefox release - Firefox 39.0 currently. https://www.mozilla.org/en-US/firefox/all/

Just install it over your current installation. Keep in mind that making a "jump" of 29 versions from Firefox 10 to 39 might present an issue or two with regards to your Firefox data files.

Note that updating via multiple intermediate Firefox versions until you reach the current release would allow to run migration/conversion code to handle differences in the format of some files as there have been changes. That would minimize possible dataloss caused by incompatible files.

So make sure to backup at least the bookmarks via an HTML backup and other valuable data like the names and passwords.