Do they display correctly if you reload the page? I've noticed some issues with this recently, having to reload some pages, but I can't figure out the pattern.

No, they don't display correctly again until I restart Firefox. Then, again, they will only disply on the first page ...

Could you test in a new Firefox profile?

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, close Firefox and fire up the Profile Manager: Managing profiles.

If the pages load correctly in the new profile, you could import key settings like bookmarks from your problem profile to the new one. More info: Recovering important data from an old profile.

@ jscher2000 Thank you very much — that seems to have done the trick!

Sadly the correction was short-lived. I reset the profile via VPN, so I wasn't able to play around with it for too long (being at work). After around 6 pages of a single site, the display returns to HTML again.

Change to another site and everything is fine for 6 or so pages. If I quit Firefox and restart, I can start over. The site that was only displaying HTML before will now display CSS again for 6 pages or so ... Do you have any further suggestions?

I can't think of any good reason for it.

I'm assuming Safari works normally on the same sites. Are there any differences in the way you connect between the two browsers, such as proxy servers or security scanners?

Safari and Opera both display the same pages normally. The connection process is the same in all three cases. There is no proxy server and security settings via virus barrier have remained unchanged for many months (since before the problems began)

Reload web page(s) and bypass the cache.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Cmd + Shift + R" (MAC)

Do you have cleanup software that might corrupt the browser cache?

@cor-el As I wrote in my first post — my problems arose, when I cleaned my caches — with 'Main-Menu' for Mac.

I'll try bypassing the cache as soon as I'm back at my machine, but that can only be a workaround, not a solution?

How old was the version of MainMenu that you ran? The current download contains files updated on June 26th, so presumably it is aware of the huge changes in Firefox's cache folders that started in Firefox 4.

If you are working in a new profile (each profile has separate cache folders), and have not let MainMenu touch the cache folders in the new profile, then MainMenu probably isn't a factor.

Main Menu was at its previous version when I deleted the caches. But then, so was Firefox (4.x). Strangely, updating to Firefox 5 did not help.

Command+R does refresh the page complete with its CSS-Settings, but, as I said, this is a workaround but not a solution.

Yesterday I deleted everything from my Mac that was Firefox or Mozilla. I then reinstalled Firefox 5. Same results — after displaying 6 pages normally, CSS was suppressed and HTML was displayed. Fairly obviously the problem lies somewhere within the system but Safari and Opera display normally. Does anyone have any idea what could have been corrupted?

Got the same problem (Windows 7, Firefox 5) and Shift + Refresh button solved it. After that, sites apear normaly.

I just want to shed some more light on how it happened, maybe it will be usefull for a solution. I ran CCleaner while surfing with Firefox, and CC got stuck for a while while cleaning Firefox cache. I still had Firefox open, didn't close it. After some time CCleaner finished cleaning (removing cache among other stuff) and immediately I had problem with Firefox not showing CSS, only plane html.

Then I saw this thread and tried above mention solution. So it is neccessary to bypass cache for every previously broken site, but after that, sites apear normal again. And sites that were not cached (and subsequently not altered by cache cleaning) never had problems.

@Mirza13 - Thanks for your insight. Nice to know that someone else has had the same problem. I'm glad the workaround works for you. In my case, bypassing the cache only works for the current page. The following pages are still displayed without CSS.

I'm also having this problem with Firefox 5 on Windows 7. I haven't used CCleaner or Main Menu. I have been Shift + Refreshing sites to get the CSS to display but it doesn't seem to be a permanent fix. Pages previously refreshed eventually need to be refreshed again though it's much much greater than 6 views for me. I'm not seeing a pattern. It happens to both sites I've been to many times and others that I've never visited before.

Just throwing this out there to show the issue is more widespread.

I'm wondering if this bug has been addressed yet? I've been having this issue with Firefox on my machine at my office, but not at home. We've looked at DNSing issues, to no avail. No other browser has this issue. I'm now running FF13 (just updated) and the issue remains.

F5/shift-F5/cache clearing/etc - all are merely temporary fixes. Randomly it seems, some sites will not load/display CSS formatting. After a few page refreshes (hard or soft) the CSS will load.

I'm if on the ball, I'll load my http tracker to find out what's being requested, and it seems that when a page loading incorrectly is being requested, FF isn't even actually -requesting- the CSS files. http requests are made for other source files, even requests that direct the browser to use the cached version (304 Not Modified).

It's as if the browser is simply ignoring included CSS files, for a few seconds, before finally realizing 'hey, I need this'. We haven't been able to track down any solutions. Other reports have resorted to the shift-F5 solution, but without any followups, so I don't know if it's the same issue or it occurred for them again but they didn't return to update the report.

This support thread is about the closest I've found to the situation I've been trying to resolve. Any further insight into what's happening here?

In Firefox you need to use Ctrl+F5 to bypass the cache (Shift+F5 is for IE and doesn't have extra effects; in Firefox same as F5 refresh).
You may still have routers or other caching going on (ISP or router?) if you need to reload pages that often.
You normally only need to do that if files on the server were updated and the server doesn't send correct response headers.

Sorry yes, my point was hard vs soft refresh - whether it's F5, Shift+F5, Ctrl+F5, physically emptying the cache, etc. Nothing resolves the recurring issue.

Another point we'd considered -- in setting the DNS servers explicitly on my computer (bypassing our internal DNSing), the issue seemed to be resolved. But we need our machines to resolve through our internal servers as we have a lot of necessary internal routing.

And again, this -only- happens in Firefox, no other browser. So it seems to be related to only Firefox, and something between the browser and the internet beyond our servers.

Your comment about server/router caching is something that came to mind, but why would that only affect Firefox? And if it is only affecting Firefox, then what setting can be adjusted or removed (whether in the browser or on a server) so that it works, like all the other browsers?