I bought a C6280 a few weeks ago, here are a few notes on how to get it working over Ethernet on Slackware 12.

Slackware 12 comes with an HPLIP package version 1.7.4a, however, according to the HPLIP home page, you need at least 2.7.9. There is an HPLIP 2.7.10 package in slackware-current; I tried this, but I couldn't get it to pass hp-check, kept complaining about not seeing the python extensions (like cupsext).

I ended up building a package for HPLIP 2.7.12 from the source code, configuring it with the following options:
./configure --prefix=/usr --disable-foomatic-xml-install --enable-foomatic-ppd-install
--with-hpppddir=/usr/share/ppd/hpijs/HP --disable-pp-build --disable-scan-build
--disable-gui-build --disable-fax-build --enable-network-build

I used trackinstall from src2pkg to build the package (thanks, gnashley!)

hp-check and hp-makeuri worked as advertised. I also installed the following packages from slackware-current: cups 1.3.4, ghostscript 8.61, and foomatic 3.0-20070919. The cups and ghostscript packages from Slackware 12 would probably have worked just fine, but there was no foomatic package in Slackware 12.

The printouts from Firefox and vi look great, color-wise.

The main problem I need to fix is the margins are messed up (In Firefox the header is cut in half and in vi the text also appears to be shifted upward). I haven't determined whether this is an HPLIP, CUPS, or application problem. Can anyone corroborate these findings? Tips?

I have the same problem with CUPS, HPLIP 1.7.4a and an HP8250 printer on Slack 12. It seems to be an issue both in Firefox as well as when printing pdf's, so I don't think it's an app thing. Haven't sorted it out yet.

The page margins in Firefox can be adjusted using File -> Page Setup... -> Margins & Header/Footer.
If you want to change the default printer settings in Firefox, type 'about:config' in the Firefox address bar, and scroll down to the 'print.XXX' settings.

If the OP and I are talking about the same problem, it isn't a margins thing; I've played with them before. What it seems to be is applications placing header information outside the printable area of the page. for instance, if you mess with the margins in Firefox, it scales the body of the document accordingly but leaves the header way up at the upper edge of the page.

With my install, under 'about:config' I find printer specific settings like these.

I took a look at these settings in my instance of Firefox (2.0.0.11). I don't have any called print_edge*, but I do have print_margin* and the value is 0.5 which I think is the default. I'm still trying to understand how to best troubleshoot this problem, but I think I'm with xflow7 in that I don't think it's necessarily a margin problem, but I don't know what a better name is.

I did find two settings that seem interesting:
print_paper_height 279.40
print_paper_width 215.90

These are for print_paper_name of Letter. A few questions,

(1) Are these settings passed to Firefox from CUPS?
(2) Are they supposed to match the values on the test page from CUPS? In the "Imageable Area" on the CUPS test page, it claims the letter page size is 209.6x273.1mm, which is smaller than what Firefox seems to think it is.

I suggest that you try changing the print_paper_height and print_paper_width from the absolute size of the Letter paper ( 279.40x215.90 m ) to the printable size ( 273.1x209.6mm ) in Firefox.

I tried tweaking with the options in Firefox, but the values didn't seem to affect the printouts at all. I also submitted a bug report to HPLIP support this week, the report is at https://answers.launchpad.net/hplip/+question/24362. Given the comments from Aaron in that report and the fact that I was able to reproduce this problem in Firefox on a different printer (HP 2600n), different version of CUPS (1.2.11), doesn't use HPLIP (uses foo2zjs), I think evidence points to an application problem, not a problem with CUPS or HPLIP.

See the Firefox printing bug reports at https://bugzilla.mozilla.org/show_bug.cgi?id=130075 and https://bugzilla.mozilla.org/show_bug.cgi?id=414314. It sounds like this and related printing issues are known, but no one is actively working on it right now.

Aaron does mention a workaround involving hp-toolbox that I haven't tried yet. I'll probably try this weekend. Thanks to all for the helpful suggestions so far.