Comment 7 for bug 229952

It seems to me that what you are actually complaining about is the way Openoffice handles read-only files. The proper way to fix this is to change the OpenOffice behaviour. Making firefox pretend to OpenOffice that the file is writeable may work around this particular problem, it it is bound to introduce new ones. So personally I would report this as a bug in OpenOffice. Many other programs allow to edit read-only files.