You could make a rewrite extension that seeks all video elements to 30s by default. Might not be perfect but it sounds like it would be useful for you.
For example: https://addons.mozilla.org/en-US/firefox/addon/re-enable-rig... is a Firefox extension that 'rewrites' webpages to remove 'disable right click' code, https://addons.mozilla.org/en-US/firefox/addon/disable-autop... 'rewrites' webpages to disable HTML5 video autoplay, etc. So you could, perhaps, rewrite the <source src=""> attribute within <video> elements to, if it doesn't already end with #t=x (x is in seconds), to end with #t=30. (Perhaps only the first one? This isn't something I desire, so my investment is limited.)
One should really avoid installing a bunch of random extension with read/write permissions to every page. Both of these extensions have ~quivalents within firefox' settings:
media.autoplay.*
dom.event.contextmenu.enabled
Firefox also allows you to manage autoplay permissions on a per-page basis via regular settings (I believe it also shows via a setting on the URL bar if firefox blocked autoplay).
Also Shift+RMB will bypass any contextmenu blocking even when it's enabled by default.
If you want to fix unselectable text, you could also use the adblocker you already have to achieve that. For example using these rules:
*##+js(acis, disableSelection, reEnable)
*##+js(acis, document.oncontextmenu)
*##+js(aopr, document.ondragstart)
*##*:style(-webkit-touch-callout: default !important; -webkit-user-select: text !important; -moz-user-select: text !important; -ms-user-select: text !important; user-select: text !important;)