Saving Word, Mozilla and other documents as PostscriptPostscript is not, strictly speaking, a file format: it's a typesetting language. Consequently, you will not find it listed under the Save As... options in applications such as Microsoft Word, OpenOffice, Firefox, Adobe Reader and so on. If you want to save a file as Postscript (perhaps to print later) most applications support this but it is under the Print options rather than the File options.
When you click on Print... in any of the above applications you will see that under the option to select a printer, there is also an option to print to file. This may be a data entry field so you can enter the name of the file; often it's just a tick box which, when selected, prompts you for a file name. Either way, when you click on Print or Save the Postscript that would have been sent to a printer is instead written to the file you've specified. You can view this Postscript file with gv to verify it's the same file/webpage you were just looking at but if you look at the file with an editor, for instance, you'll find it's unreadable as it's now embedded in Postscript commands
Most applications either allow you to enter the complete file name including suffix, or set it to .ps. Microsoft Word has to be different and by default will set the suffix to .prn. If you select All file types from the drop-down menu then you can enter your own suffix.