Do you mean you want websites to format times as though you are British? You could try sending "English/United Kingdom" (en-GB) to see whether that gets pages displayed the way you want. You can set that here:

"3-bar" menu button (or Tools menu) > Options > Content

Click the "Choose" button to open the Languages dialog, and use the drop-down control at the bottom to add "English/United Kingdom" or another English variation of your choice. You might need to manually move it up the list if it isn't added at the top.

Does that work??

Note that for sites you've already visited, you may need to clear cached pages from the site or perhaps even cookies storing your previous language preference.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

"3-bar" menu button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page.

Hi jscher2000, Thank you for your response. The answer is....sort of. I tried using every possible language setting but none of them accomplished what I need. The problem with the [en-GB] language setting is that while it does provide the time in 24-hour format as needed, it uses European date format of dd/mm/yy, which can be confusing when you are used to seeing dates in the format of mm/dd/yy. The [en-GB] language setting also change some of the text to the GB versions of words (e.g. colour instead of color).

As far as I can tell, the only way to accomplish what I need would be to create a new language pack that is essentially a direct copy of the [en] language but with the time format being changed to 24-hour. The problem is that I have no idea how to accomplish this or if it would be a nightmare to maintain as Mozilla releases new Firefox versions. Any thoughts?

Oops, I assumed full European style. Sorry about that.

The issue is that this is not under Firefox's control. Instead, websites render dates and times in a format they think you will prefer based on your language preference or location. If none of the existing language-locale pairs triggers the right behavior, it's unlikely that you could improvise one that works because it would be unknown to the site.

Some websites may offer the option to change from an AM/PM clock to a 24-hour clock in your account settings (for example, bulletin boards often do this), but if you do not log in, then that wouldn't work.

It's possible to "hack" webpages to reformat what you see on the screen using an add-on. For example, I worked on a Greasemonkey userscript that converted the other direction in this thread: http://userscripts-mirror.org/topics/101035.html. I think your request is much simpler, actually.