i would take a punt at computer related - for the following reasons:
if it were site related, it would do it for everyone using xx or yy browser (different browsers interpret html differently, hence the variation between them), but all copies of firefox 39 will interpret it the same, so it should be shared across the same browser.
if it were browser related, uninstalling, and reinstalling the software would almost certainly fix it, and as above, if it were an issue with a specific browser, anyone using that browser would have the same issue.
The only thing that leaves is the computer itself, although im not sure what exactly it could be, a missing file? not likely. maybe its something to do with running the editor that does something (I assume its an editor that runs in the browser window and not a stand alone program?).
out of interest, if you start in private browsing, and navigate to the site, does the site work? (CTRL+SHIFT+P). this would take a cookie/cache issue out the equiation as well as these are not stored during a private browsing session. worth a shot since its easy to try. I would assume the problem will still persist during this though, which would be further evidence it is somehow computer related - purely because you have the same issue for other weebly sites you have visited on those computers.
if it were site related, it would do it for everyone using xx or yy browser (different browsers interpret html differently, hence the variation between them), but all copies of firefox 39 will interpret it the same, so it should be shared across the same browser.
if it were browser related, uninstalling, and reinstalling the software would almost certainly fix it, and as above, if it were an issue with a specific browser, anyone using that browser would have the same issue.
The only thing that leaves is the computer itself, although im not sure what exactly it could be, a missing file? not likely. maybe its something to do with running the editor that does something (I assume its an editor that runs in the browser window and not a stand alone program?).
out of interest, if you start in private browsing, and navigate to the site, does the site work? (CTRL+SHIFT+P). this would take a cookie/cache issue out the equiation as well as these are not stored during a private browsing session. worth a shot since its easy to try. I would assume the problem will still persist during this though, which would be further evidence it is somehow computer related - purely because you have the same issue for other weebly sites you have visited on those computers.