I use Win 10 Firefox 59.0.2 64bit and even in Safe Mode images and gifs load very very slowly, if at all in Tumblr.
I disabled hardware acceleration and accessibility . In about: config, the image animation mode is already set to normal. Reloading the image sometimes, sometimes not works. Opening in another window or tab is equally slow, safe mode or not. Tumblr is even slow loading before I even log in, safe mode or not.
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One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}
In safe mode firefox indicates there are no tracking elements detected on Tumblr so there is no shield. The lock is green indicating a secure connection. When not in safe mode I have disabled tracking protection but images and gifs still do not load smoothly.
Please try : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files , C:\Program Files(x86) & C:\ProgramData Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :
- https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
- https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
Reinstall with Current Release Firefox 59.0.2 with a Full Version Installer
Please let us know if this solved your issue or if need further assistance.