Is there a way to disable/switch a plugin depending on the user OS/browser?

Question

For instance to disable wysiwyg/ckeditor plugin or to use the basic q2a editor on IE v. 8 or less on a Windows Xp machine...

Answer 1

You would be best off making a custom plugin that combines those two plugins. For example make a copy of the WYSIWYG plugin, then in the functions like get_field, do

if (/*code to check browser is IE < 8 */) {
    // code for plain editor
}
else {
    // code for WYSIWYG editor
}