furtherance (n.)

Old English fyrðran, fyrðrian "to impel, urge on; advance, promote, benefit;" see further (adv.). Compare Middle Low German vorderen, Old High German furdiran, German fördern, probably from their respective adjectives via the notion in phrases such as Old English don furðor "to promote." Related: Furthered; furthering. After the further/farther split, this sense also continued in a shadow verb farther (v.), attested from 16c. but apparently dying out 19c.

word-forming element attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from Latin -antia and -entia, which depended on the vowel in the stem word, from PIE *-nt-, adjectival suffix.

Latin present-participle endings for verbs stems in -a- were distinguished from those in -i- and -e-. Hence Modern English protestant, opponent, obedient from Latin protestare, opponere, obedire.

As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence,absence).

English thus inherited a confused mass of words from French (crescent/croissant), and further confused it since c. 1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.