I have same problem in win7.
many years ago you could install seamonkey WITHOUT email and could have many 'windows' [wrong term?] open on different html files which could be edited and saved correctly. Recently the option to install without email has gone and seamonkey uses tabs. The result is that when first open one can edit/save an html file, but (I understand) if open a second file for edit then save fails and if use 'save as' it asks for a new filename which works, but saveing over the old file fails.
I have noticed if start seamonkey from cold, if you click on a (html) file, then seamonkey starts, but if then click on a second html file the result is that a new tab opens in seamonkey rather than a new instance of seamonkey. I'm an old man and dont really understand these things, but it occurs to me an option to open a new file as a new instance of seamonkey rather than open in a new tab then it may help.
Thanks - seamonkey is one of my few main tools.
angus