1. Enable Mozilla plugin by checking Tools->Options->Internet->Mozilla Plugin in OpenOffice Writer.
2. Check whether you’ve got the OpenOffice-Mozilla Plugin at /usr/lib/openoffice.org2.0/program/libnpsoplugin.so
3. Create a symbollic link at /usr/lib/mozilla/plugins/ to the above
libnpsoplugin.so to enable mozilla-openoffice plugin in firefox. You
need to restart firefox after this step.
4. Edit ~/.mozilla/firefox/pluginreg.dat for the following changes:
4.1 Search for the following text

OpenOffice.org Plug-in handles all its documents:$<br/>
OpenOffice.org Plug-in:$
31

4.2 Note the italicized count.
4.3 To the end of the list add the following text

31:application/msword:Opendocument Text:doc:$
32:application/vnd.ms-excel:Opendocument Spreadsheet:xls:$
33:application/vnd.ms-powerpoint:Opendocument Presentation:ppt:$

4.4 The index of the left of the above text should be relative.
4.5 The count should be updated as count = count + 3 ( as you are adding 3 more entries )
5. Restart firefox
6. Try opening a document in firefox; It will take few extra seconds to
start but eventually it will open the document inside firefox brower
itself.

A typical openoffice section would look like the following:

/usr/lib/openoffice.org2.0/program/libnpsoplugin.so:$
:$
1160152037000:1:5:$
OpenOffice.org Plug-in handles all its documents:$
OpenOffice.org Plug-in:$
34
0:application/vnd.stardivision.calc:StarCalc 3.0 – 5.0:sdc:$
1:application/vnd.stardivision.chart:StarChart 3.0 – 5.0:sds:$
2:application/vnd.stardivision.draw:StarDraw 3.0 – 5.0:sda:$
3:application/vnd.stardivision.impress:StarImpress 3.0 – 5.0:sdd:$
4:application/vnd.stardivision.impress-packed:StarImpress-packed 3.0 – 5.0:sdp:$
5:application/vnd.stardivision.math:StarMath 3.0 – 5.0:smf:$
6:application/vnd.stardivision.writer:StarWriter Template 3.0 – 5.0:vor:$
7:application/vnd.stardivision.writer-global:StarWriter Global 3.0 – 5.0:sgl:$
8:application/vnd.staroffice.writer:StarWriter 3.0 – 5.0:sdw:$
9:application/vnd.sun.xml.calc:StarOffice 6.0/7 Spreadsheet:sxc:$
10:application/vnd.sun.xml.calc.template:StarOffice 6.0/7 Spreadsheet Template:stc:$
11:application/vnd.sun.xml.draw:StarOffice 6.0/7 Drawing:sxd:$
12:application/vnd.sun.xml.draw.template:StarOffice 6.0/7 Drawing Template:std:$
13:application/vnd.sun.xml.impress:StarOffice 6.0/7 Presentation:sxi:$
14:application/vnd.sun.xml.impress.template:StarOffice 6.0/7 Presentation Template:sti:$
15:application/vnd.sun.xml.math:StarOffice 6.0/7 Formula:sxm:$
16:application/vnd.sun.xml.writer:StarOffice 6.0/7 Text Document:sxw:$
17:application/vnd.sun.xml.writer.global:StarOffice 6.0/7 Master Document:sxg:$
18:application/vnd.sun.xml.writer.template:StarOffice 6.0/7 Text Document Template:stw:$
19:application/vnd.oasis.opendocument.text:OpenDocument Text:odt:$
20:application/vnd.oasis.opendocument.text-template:OpenDocument Text Template:ott:$
21:application/vnd.oasis.opendocument.text-master:OpenDocument Master Document:odm:$
22:application/vnd.oasis.opendocument.text-web:HTML Document Template:oth:$
23:application/vnd.oasis.opendocument.spreadsheet:OpenDocument Spreadsheet:ods:$
24:application/vnd.oasis.opendocument.spreadsheet-template:OpenDocument Spreadsheet Template:ots:$
25:application/vnd.oasis.opendocument.graphics:OpenDocument Drawing:odg:$
26:application/vnd.oasis.opendocument.graphics-template:OpenDocument Drawing Template:otg:$
27:application/vnd.oasis.opendocument.presentation:OpenDocument Presentation:odp:$
28:application/vnd.oasis.opendocument.presentation-template:OpenDocument Presentation Template:otp:$
29:application/vnd.oasis.opendocument.formula:OpenDocument Formula:odf:$
30:application/vnd.sun.xml.base:OpenDocument Database:odb:$
31:application/msword:Opendocument Text:doc:$
32:application/vnd.ms-excel:Opendocument Spreadsheet:xls:$
33:application/vnd.ms-powerpoint:Opendocument Presentation:ppt:$

When I was writing this, the following are the version numbers:

1. OpenOffice.org 2.0.4-5.3
2. Mozilla Firefox 1.5.0.10