Bug 28904: add specific settings for 8.0
Bug 28904: add specific settings for 8.0

Remove variable specific settings for 7.0 and add one for 8.5.

We also remove the general.useragent.override pref which is not set
anymore with the changes for #26146.

boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite