I tried to reproduce the issue but without any success. Build on which i done the testing, Firefox Release 68.3.0 with OnePlus A5000 (Android 9), OnePlus 6T/A6013(Android 9) and with Moto G4 (Android 6.0.1).
S1 - Tried to copy text from IMDB on a synopsis page.
- S1.1 - copied a word
- S1.2 - copied a text
- S1.3 - link or other page elements included in the copied text
- S1.4 - made a select all action
S2 - Opened a link to twitter with Firefox Release set as default no crash encounter through external apps. I did some browsing on twitter, made some searches, opened some images.
S3 - Private Mode - Opened a link from twitter, viewing other tweets, copied a twitter link and made a search after it, long tapped on a link and open it through "Open Link in New Tab" and "Open Link in Private Tab" in private browsing, all of this by being logged with a twitter account in normal browsing.
S4 - Logged on Twitter on private mode-open link from a tweet, opened huffpost.com article through twitter.
S5 - Tried to open through twitter, a couple of links with articles that will redirect to huffpost.com.
Did some exploratory played a video being logged in twitter, had multiple tabs with tweets opened, set the video in fullscreen.